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bglick
28-Jul-2005, 22:55
Help me understand this.... I am trying to find a method to focus, without bellows or any movements and helical mounts are not possible (not worth getting into that part)

I beleive a Helical focussing mount simply provides greater distance between the front and back lens sections, which appears is seperated by the shutter. (at least in View Camera lenses) If this is correct, and I am using a 47mm focal lenght lens...and only need focussing between infinity and say 20 ft..... then my calculations say the additional spacing requried between nodal point and film plane is only .3mm. Now, I don't know how this equates to the lens element seperation when focussing with a helical mount? But I assumed it would be close...or at least not much differences, as the helical mounts themselves are not very thick.... So I took a 55mm Rodenstock Grandagon and put it on my view camera.... I focussed at infinity...then tried to unscrew the front element, making seperation between the front and back element. I thought I would be simulating a helical focussing mount. But much to my surprise, even when unscrewing the lens front all the way out, at least 4mm, I noticed the focus point on the gg was not changing? What is flawed here? This would be a very elegant solution.
Possibly I need a longer threaded barrel to screw into, i.e. to create more distance?

Next I thought about shims on the back, as the lens board can't be moved....but this would be cumbersome, as I need shims in the .1mm thickness range, which would be hard to keep them from bending as they would need to fit around a 5x7 back. So I really want to explore this pseudo helical concept as it will be the easiest method to focus in the field as I do have gg to view focus point. And since my focus requirements are so small, I would think there must be some mechanical solution?

TYIA

Doug Dolde
28-Jul-2005, 23:08
A helical focusing mount moves the whole lens. Let me have some of what you're smoking dude.

Richard Ide
28-Jul-2005, 23:16
Bill

"Help me understand this.... I am trying to find a method to focus, without bellows or any movements and helical mounts are not possible (not worth getting into that part"

I don't think you can focus ( a camera anyway ) without movements of some kind. What exactly are you trying to do?? A little more information will reduce the circle of confusion.

Richard

bglick
28-Jul-2005, 23:38
Doug, thanks for you kind response.... I guess I am smoking stupidity. I never used or saw a helical mount before and I thought this forum was for learning? If everyone knew everything, there would be no questions.

However, from the looks of them, and the way internal focus mounts work on MF cameras, I never saw the entire lens move to focus. Certainly not true for Mamiya 7 lenses. Can anyone confirm this?

Anyway, if Doug is right, then my pseudo helical concept is shot....but it did give me an idea, i.e. to custom make a fitting to go inside the lens board hole, and screw the entire lens in and out....... since I only need .3mm, some tight threads might do the trick.

Rich, in short, I am building a camera whereas there the lens is fixed and no movements possible, so I am trying to come up with a method to focus the lens on the gg.

John Berry ( Roadkill )
28-Jul-2005, 23:48
focus = movement. The only option is fixed hyperfocal and heaps of f-stop

Richard Ide
28-Jul-2005, 23:53
Bill

Just put shims behind the lens, but on the front of the lens board. If you need movement the other way, you will have to reduce the lensboard thickness. Shim stock, either brass or steel is available from .oo1" ( .0254mm ) and thicker. Try asking a machine shop if they can supply your needs.

Richard

Richard Ide
28-Jul-2005, 23:56
With 35mm and MF lenses, various elements in the lens move in relation to each other when you rotate the focussing ring.

Paul Fitzgerald
29-Jul-2005, 00:21
Bill,

Helical focusing moves the whole lens, front focusing lenses were used on folders from the 40's - 50's. By moving the elements apart it actually changes the focal length of the lens and moves the focus point.

50mm was for 35mm film; 75mm was 6x6; 105mm was 6x9; there are plenty on ebay if you want to try it out.

The antique wet plate cameras had the lenses mounted tube-in tube and moved quite a distance, a simple set screw would hold position. A helical slot thru the outer tube and the set screw to the inner would provide movement, no gear train needed.

Good luck with it.

bglick
29-Jul-2005, 01:36
John, "focus = movement. The only option is fixed hyperfocal and heaps of f-stop"

I did strongly consider this, but its not the ideal set up, last option for sure.....

Richard, I think moving the lens (in the lens board) would be easier in the field vs. using shims of such tiny thicknesses... And yes, of all the non LF lenses I have, I never saw the entire lens move as Doug stated.... but obviously on LF lenses the helical moves entire lens.

Paul... "A helical slot thru the outer tube and the set screw to the inner would provide movement, no gear train needed."

I keep reading this but don't get it. What's wrong with a simple threaded fitting going through the lens board.... a simple twist of the lens, maybe one revolution, will give me the full .3mm focus adjustment I need? Is your concept just a simple push/pull, like telescoping rail? Sorry I am not seeing the advantage, can you please elaborate... TYIA

GPS
29-Jul-2005, 03:13
"A simple threaded fitting" - needs to be firmly and precisely attached to the lens board so that the movement doesn't change the precise position of the lens that must be strictly parallel to the film plane. This is why helical focusing mounts are expensive - they are a piece of fine mechanics. And yes, even helical focusing on 35 mm SLR moves the whole lens... Your concept of "in between" focusing lens cells is indeed an idea of a smoker.

Ole Tjugen
29-Jul-2005, 03:25
May old portrait cameras (and lenses) have rack-and-pinion focusing. That might be worth looking into?

Considering the money that has been spent on advertising "internal focusing" in 35mm SLR lenses, it is a little too simple to make a blanket statement that "helical focusing on 35mm SLR moves the whole lens". It does for most lenses, but far from all.

GPS
29-Jul-2005, 04:01
Ok, zooms and some more special lenses (achromats) have the internal focusing but, as you say, most of "normal" SLR lenses move the whole assembly. Sometimes it's difficult to see as the lens can be burried deeply in the lens tube.

Dan Fromm
29-Jul-2005, 04:53
Richard Ide wrote "With 35mm and MF lenses, various elements in the lens move in relation to each other when you rotate the focussing ring."

In general, stuff and nonsense, but there are some exceptions. I think they're irrelevant.

You're right for some crappy 35 mm, 6x6, and 6x9 cameras with fixed lenses that have front element focusing. This is widely recognized as a compromise between cost (= complexity) and quality.

In more "serious" gear, the first lens to market with a floating element, if I'm not mistaken, was the 24/2.8 Nikkor in Nikon F mount. And in that lens the floating element is there to reduce aberrations, mainly coma, at close focusing distances.

Internal focusing is a relatively new development. Lenses for LF cameras are all unit focusing.

Bill, lenses for LF cameras are designed to work best when the cells are screwed tight into the shutter. When you change cell spacing, especially with wide angle lenses, what you mainly do is add aberrations. Another way of looking at focusing is that what it really does is change magnification. Either way, something has to move to change the distance between the lens and the film plane.

Bob Salomon
29-Jul-2005, 05:40
Bill,

View camera lenses focus, as you have discovered, by moving the entire lens towards or away from the film. That is the purpose of a bellows on a view camera. It lets you move the standards towards or away from each other.

That is also the purpose for a view camera lens or an enlarging lens helicoid. It moves the entire lens torwards or away from the film.

There is also a thrid and simpler way to do this. That would require two tube, one that simply slides front to back inside the other. You mount your shutter to the front of inner tube and you mount your camera to the back of the larger tube. Then you simply slide the lens in or out till the image is in focus. To lock it in place you tighten a set screw.

Drawbacks to this system? Ain't as smooth and accurate as moving two standards on a rail or in and out via helicoid and parallisim between the lens and film is harder to maintain so sharpness overall can be effected.

Caveat - the tubes both have to be at least as large in diameter as the rear elemnet or the lens won't fit.

GPS
29-Jul-2005, 06:03
One more thing - just because your " focus requirments" are so small (0.3 mm indeed) you cannot rely on a simple straightforward movement guided by your fingers. You could focus for hours in this way without being sure where you really focus. The helical focusing ring is there to multiply those 0.3 mm on a longer scale - that of the circumference of the helical focus. Useless to try to reinvent the wheel.

Mike Gudzinowicz
29-Jul-2005, 07:35
Your introductory statement mentioned a 5x7 camera back. The
47 mm and 55 mm lenses won't cover the format... a 72 mm SSA XL
will.

If you choose the helical mount (commercial or homemade) to insure
focusing ease and accuracy, the centering of the lens optical axis
with the focusing movement and the alignment of the mount to the film
plane must be precise.

The deviation from planarity of the perpendicular line from
the lens axis to the corner should be on the order of 0.03 mm or
0.001".

You can not rely on depth of focus to "correct" that problem.
The depth of focus formula (N*c) assumes acceptable focus on a plane.
Using that recommendation, you might find that two opposing corners are
sharp, and the others are focused in front of and behind the plane, or
that one edge is focused in front of the plane, and the other behind it
when the center is in sharp focus. After mounting the lens, shim the lens
board so focus is sharp across the entire field.

If you are using a helical mount, there is a further restriction that focus
is sharp at EVERY MAGNIFICATION. In order to do that, the lens axis must be
precisely centered in the mount. Otherwise while focusing, the axis will
wander in a circle on the film plane forming a "cone" making alignment
lens board alignment impossible at different magnifications. The commercial
helical mounts provide the required lens axis alignment, though the lens board
may require shims in a homemade box camera.

If you want to use wide lenses in the manner you've suggested, the
"alignment specs" that are required are similar or should exceed those
of a good medium format camera. The expensive but easiest and most reliable
approach is to buy a special purpose camera like a Silvestri which exceeds
the requirements, or start experiments with a commercial mount or one made
by a good machine shop.

There still will be issues such as film flatness, etc. which will require
that the lens be stopped down.

Paul Fitzgerald
29-Jul-2005, 08:47
Hi there,

Bill:

("Paul... "A helical slot thru the outer tube and the set screw to the inner would provide movement, no gear train needed."

I keep reading this but don't get it. What's wrong with a simple threaded fitting going through the lens board.... a simple twist of the lens, maybe one revolution, will give me the full .3mm focus adjustment I need? Is your concept just a simple push/pull, like telescoping rail? Sorry I am not seeing the advantage, can you please elaborate... TYIA")

Close to push-pull but the helical slot reduces the movement, easier to focus. The tube-in-tube setup is easier to machine properly, no wobble when you change focus. A helix mount is VERY expensive to build precisely.

You could look for a Kodak Medalist; MedalistII shell for the focus mount, but you would have to band-saw the body away. The tube ID is approx.55mm..

You can just build it and set the focus at 30 feet and use depth of field.

On front-cell-focus folders, it only moved the very front lens element NOT the whole front cell, thats why it didn't work for you.

Have fun with it.

Paul Butzi
29-Jul-2005, 09:28
Something to consider - all you need to do is change the distance between the lens and the film. It doesn't matter whether the lens moves, or the film moves.

Can you just have a set of shims for the back of the camera, different thicknesses for different focus distances?

Richard Schlesinger
29-Jul-2005, 10:26
Zone VI at one time offered an arrangement for aligning the lens on earlier Beseler enlargers. This involved two lensboards separated by a piece of neoprene and held together with thre equally spaced screws. Tightening and/or loosening the screws changed the position of the lensboard. Not a precision arrangement but workable with GG viewing?

Neal Shields
29-Jul-2005, 15:58
I purchased a broken Contarex on Ebay that suffered from a bout with the Yugoslavian border guards where they had tried to unscrew a bayonet lens to check for boo. It was in the early 70 and most cameras in eastern Europe apparently still had screw mount lenses.

What they had actually done was force the helical mount past it's stop.

After about 20 tries and 20 squared cuss words I got it back together. There are four sets of threads, nested within each other.

What I learned is that the point of that type of mount seems to be to have an adjustable screw mount that allows the user to change the distance between the lens and the film without the lens body turning. I.E. your F stops and focus ring always stay on top.

A helical mount consists of a male set of threads inside a tube with female threads on the inside and another set of male threads on the outside, which then engages a set of female threads on the part that you turn.

The whole thing functions like a planetary gear set where if you hold one element and turn another the third turns in the opposite direction.

If you don't care if your lens turns when you screw it in and out, I see no reason that you need a "helical" mount just a normal screw mount with very course threads.

Keep in mind this is all "like triangles” a little bit of movement goes a long way on a 50mm lens but you need a lot more movement on a large format lens to do the same thing.

I suppose that any screw mount would be a “helical mount” but I think the term has come to mean what is described above at least when speaking of lens mounts.

bglick
29-Jul-2005, 16:34
Niel, great points... your description gives more credence to using rear shims as the best focus option.... Its easier to machine some shims, ( the larger thicknesses ones I described above) vs. trying to move two lenses in and out tiny amounts with all their peripheral goodies moving with them....

I am curious when putting the gg a certain distance from the lens for infinity focus, how does one know where the point in the lens this measurement is taken from? Also, if you are perfectly focussed on the gg, this would represent the very front of the film plane...shouldn't the focus be at mid point of the film thickness?

My plan for gg focussing was, use the same blank plate the 6x17 film holder uses and attatch a gg to the back of the film opening, as this is exactly where the film will lay, well at least the top part of the film thickness. Hence my question above, as the 120 film is about .1mm thick, which is a 1/3 of my total focus range of .3mm. So where should the etched plane of the gg lie, front of film, mid point of film, back of film?

JimL
29-Jul-2005, 16:39
"I never saw the entire lens move to focus. Certainly not true for Mamiya 7 lenses."

Bill, look at your Mamiya 7 lenses again - the front and rear elements move together.

As for your focussing problem, you may be able to adapt something using one of those 35mm camera macro bellows units. Tear off all the parts and attach the rack an pinion device to your own custom made lens board and bag bellows.

bglick
29-Jul-2005, 17:01
Jim, that is nothing short of brilliant! It's amazing how easy it is to overlook something this simple and readily available, which serves the same purpose. I will research this as a viable option....not sure I can grab the parts I need and put them on the front of lens board....but it's worth investigating, as I consider this more desireable than shims.

GPS
30-Jul-2005, 01:19
Trying to focus a 47 mm lens with wobbly rack and pinion (that's what they are on the 35mm camera macro bellows) on 0.3 mm - a great idea, indeed. Welcome to the real world afterwards.

Mike Phifer
30-Jul-2005, 17:59
Bill, Three tenths of a millimeter is quite small. It is about the thickness 3 sheets of copy paper. Why not use a screw flange like those on many large format lens-to -lens board mounts ,to accomplish this (a poor man's helical mount). Depending on the thread pitch, this distance would be within a half of a turn. (I tried this with my old DAGOR and it was still secure. It has a fine thread pitch: Looks to be about 50 threads per inch.)

bglick
30-Jul-2005, 20:02
I looked at some 35mm macro rigs, and could not find something suitable, mainly due to the positioning of the unit.

Mike, yes, this is what I was writing above, a high thread count adjustment, but with two lenses on one lens board, getting the board to pull in and out evenly is not easy, as Mike above stated just how precise the lens axis needs to be. But I am not ruling this out....

The front of my VX125 has a fine focus knob.... however, term "fine" is relative, as this is LF and I am dealing with very short fl lenses, often not used on 4x5. Making the 1" diam wheel larger, say 2" diam. to gain more fine finger precision, it would take a turn of .6mm on the circumfrence to gain .1mm of focus adjustment, (the min. I need). Sheeesh, even this seems very difficult, not to mention the other issues with the standards being square, parallel, etc.

I am starting to have strong appreciation for the ultimate short fl camera, i.e. a range finder camera, such as M7, Bronica, etc. I think a CNC machined box, and CNC machined rear shims is the only system that will acheive even rough focus which is all I am pursuing. Yep Helical mounts are sure worth their money.... they allow finger precision due to their gear advantage, and with short fl lenses, that is priceless. They might be my last resort....assuming I can force my lens centers from 65mm to 75mm. Of course maintaining the 65mm lens center was the motivating force behind this entire project... Arggg... Photography, a synonym for compromises.

Paul Fitzgerald
30-Jul-2005, 21:50
Hi there,

Oh, you're going to hate me!!

Why not mount the rear frame assm. on 4 fine double-thread screw mounts? That way you can change focus AND adjust plane of focus like a view camera.

Now to explain: I have a Stanley #92 mini-plane and the adjuster for the blade is a double threaded bolt = 1/4x20 - 5/16x18. As you turn the adjuster it un-threads from one while threading in with the other making it act as a 38TPI bolt. If you start with fine threads you will have ultra-fine adjustments. Extend the mounts out to the far corners and the leverage difference is from the opposite corner.

This is starting to sound like a rather silly camera, what is it for?

Enquiring minds want to know.

bglick
30-Jul-2005, 22:32
Paul.... thanks for the brainstorm

Why not mount the rear frame assm. on 4 fine double-thread screw mounts? That way you can change focus AND adjust plane of focus like a view camera.

This has been explored above.... it is possible, but not very practical for field use....

Now to explain: I have a Stanley #92 mini-plane and the adjuster for the blade is a double threaded bolt = 1/4x20 - 5/16x18. As you turn the adjuster it un-threads from one while threading in with the other making it act as a 38TPI bolt. If you start with fine threads you will have ultra-fine adjustments. Extend the mounts out to the far corners and the leverage difference is from the opposite corner.

So each time you want to focus, you have to turn 4 screws so gg is level, then re look gg, then turn 4 screws again, put gg back on, etc... you need to focus while viewing the gg, and at best have one focussing mechanism, or one for each lens (helical).... if this approach is taken, its more practical to use gg shims as I mentioned earlier and have 4 focus points, i.e. 3 shims.

This is starting to sound like a rather silly camera, what is it for?

Silly? You think this is fun? I am ready to have a heart attack stressin over how to build this thing :-)

As mentioned above, it will be a Medium Format stereo camera, something only available for sale new, by one German maker, Gilde. He does not have the fl's I want, or the lens seperation, as he used helicals...... (took the easy way out) otherwise, his \$18k price tag and 1 year wait is starting to seem very reasonable.

Here is a picture of one if your interested.....
http://gilde-kamera.de/en/zu4.html

Kevin Kemner
31-Jul-2005, 00:04
Bill,

Having designed and kitbashed a few cameras of dubious success you might want to consider having SK Grimes make a custom focus mount for you. If you root around on their website you will find the focus mounts they currently make and willingness to try new things. Grimes is one of the very best companies to deal with.

I've tried several times to get around the helical focus mount but for applictions like yours you can't beat it. Oh, FWIW, its called a helical because of the helicoid threading that changes pitch as the lens is moved from far focusing to close focusing.

Good luck with your project. You've inspired me to spent a little bit of time with one of my more quixotic efforts, a 16x 20 pinhole.

Kevin

David Van Gosen
1-Aug-2005, 15:27
Here's a similar project:

http://www.webstar.nl/~job/homemadepg.html

Good luck!

David

Dan Fromm
1-Aug-2005, 17:15
Bill, I've been following this thread with rising dismay and hilarity.

You'd have a better chance of getting good advice if you told us what you were trying to accomplish instead of asking for help in making a basically cockeyed design work.

So, what kind of a camera are you trying to make? Don't tell us about what seems like a bad design choice, tell us about what the finished product is to be. Tell us what you want y'r camera to do, not how you want it to go together.

Don't knock focusing by moving a standard that holds a lens along a track. It works for all of us here, and it works for me too with a small pile of bellows that I use for critical close focusing on 35 mm.

And don't knock ready-made focusing helicals. I have a couple lying around that came from 4x5 aerial cameras, and what's good enough for that application is good enough for anything.

Or are you just looking for nice people who'll try to help you no matter how dumb and helpless you make yourself look?

bglick
1-Aug-2005, 18:01
Hi Dan, this thread has become a combination of collaboration, education, and design considerations. All the while, it helped me reach the right conclusions about this camera I plan to build.

However, I always try to respond to anyone who took their time to contribue to a thread I start....so I will answer you questions, most by just "cut & pasting" from my previous posts above....

> So, what kind of a camera are you trying to make?
> Don't tell us about what seems like a bad design choice, tell us about what the finished product is to be.

From above.... (I even gave you a link to see one?? how much more descriptive can I be?)

...... it will be a Medium Format stereo camera, something only available for sale new, by one German maker, Gilde. He does not have the fl's I want, or the lens seperation, as he used helicals...... (took the easy way out) otherwise, his \$18k price tag and 1 year wait is starting to seem very reasonable.

Here is a picture of one if your interested..... http://gilde-kamera.de/en/zu4.html

> Tell us what you want y'r camera to do, not how you want it to go together.

Same thing all cameras do, take a picture, focussed at some point from the lens.

> Don't knock focusing by moving a standard that holds a lens along a track. It works for all of us here, and it works for me too with a small pile of bellows that I use for critical close focusing on 35 mm.

As I wrote several posts above, too many posts to cut and paste. I have rail / field cameras from 6x9 .... 8x10, and have used this focus method all my LF life. Never knocked it one bit. In general they work wonderful for LF, but when you start using very short fl lenses (47mm as above), and try to focus at distances which require .1mm increments of lens movement, you have reached the limits of rail camera focus... now you need a very large mechanical advantage, which this thread was exploring, and hence the beauty of helical mounts began to surface, at least for me.

> And don't knock ready-made focusing helicals.

knock em? Sheeeesh, Once again, from my post above.....

Yep Helical mounts are sure worth their money.... they allow finger precision due to their gear advantage, and with short fl lenses, that is priceless.

> I have a couple lying around that came from 4x5 aerial cameras, and what's good enough for that application is good enough for anything.

Huh? Comparing infinity focus from an airplane, vs. focussing at 15' vs. 18' subject from the lens?

> Or are you just looking for nice people who'll try to help you no matter how dumb and helpless you make yourself look?

Huh? Dan, no need to slander me, I am a pro photographer / engineer, but I never built a camera before, as not many people have. In a short period of time I have learned a lot, thanks to many people who like to collaborate on these issues. If I appear dumb and helpless, well, thats what happens when one asks a question about something they have no experience with, i.e. helicals. I thought this was how you learn? And I also thought it was the purpose of this forum? If I am wrong, please explain the purpose so I can particpate more to your liking.

The next post I will have an update..... I would hate to mix it with this response.

Thanks for contribution Dan.

bglick
1-Aug-2005, 19:30
First, thanks for all those who offered help, it's one of the nice things I enjoy about LF community, people like helping and collaborating about LF issues. Here is an update......

Getting detailed specifications from Schneider, I have found the the flange portion (lip) of their helical mounts can be scaled back a bit where the two lenses meet, allowing 69mm lens centers. I am willing to compromise the 4mm vs. my initial goal of 65mm lens centers, as the benefits of factory mounted helicals are invaluable due to the close focus positions the camera is used for.

I plan to make two stereo lens boards, with factory paired 47mm and 80mm digitar lenses, mounted on Toyo 158mm lens board. The design I finalized will allow the lens boards to serve on two camera platforms.

First, the lens boards can be put on a view camera front standard w /bag bellows which is being custom made by camerabellows.com. I have purchased from Kieth Canham, a MCQ 5x7 back. This back will be mounted on a custom " U" adapter to adjoin it to a Toyo Rail clamp. The forward placement of this back on the rail clamp is what will allow for the 47mm lenses to get close enough to the gg, something not possible with 5x7 cameras. The lens will have tilt capabilities, albeit not that much as the digitars have limited image circle, but considering how little tilt is requried for what I need, it will probably work fine...if I have light fall off, I will use ND center fitlers. To maintain lens standard parelellism, I plan to make a measuring rail, that can confirm rail spacing, at low point and high point, as Mike mentioned above. This is for slow shooting of landscapes through the gg.

Secondly, the stereo lens board can be affixed to a custom made body used for hand holding shots. It will consist of a handle, graflock to hold 6x17 back, built in septum, viewfinder for each lens fl, dual flash shoes, light meter shoe (digisix), lens protector (rail type) and spacers for infinity focussing for each fl lens used.

Lens sync as of now, will be attatched to the lens board via Degroff air solenoids. The hand bellows and tubing will be removable at the lens board.

Well, it all looks good on paper, now we shall see if it all comes together....hopefully in a few months, I will find out! Thank you all for contributing.

Dan Fromm
1-Aug-2005, 21:25
Bill, thanks for the reply. Why invent when the ideas you need are out in the open waiting to be used?

See, e.g., http://phsne.org/stereocameras/35mm-rollfilm/35mm-rollfilm.htm . Also http://www.oz3d.info/sscc/secure/3D-History.PDF There's a long list of stereo cameras that focused by moving one or a pair of lensboards. Use Google to find more.

For what you want to do, concentric boxes will do for moving the lenses forwards and back and keeping the their axes normal to the film plane.

A limiting factor that may force you to wider inter-lens spacing than you want is the diameter of the lenses' shutters. But then, that's because you're thinking of lenses in big shutters. Lenses that will do for you were sold in little ones too. I just measured the #00 Prontor Press my 47/5.6 SA is in, it is a little over 60 mm o/d. So two of them would work, and #00 Compur Rapids are even smaller.

And if you start from a severely hacked up 4x5 Speed Graphic, you can probably avoid using lenses in shutters.

Now, why did you rule out the old standard solutions to the problem?

bglick
1-Aug-2005, 22:17
Dan

> Why invent when the ideas you need are out in the open waiting to be used?

Well, thats just it, I only "re-invented the wheel" for a few components and got the benefits of using all off-the-shelf, well designed modern products.

> There's a long list of stereo cameras that focused by moving one or a pair of lensboards. Use Google to find more.

Dan, most all of these cameras were made in 1800's - 1940's. During these times, film and optics did not half the resolution capability of todays modern films and optics. Therefore precision focus was not as critical. Not to mention most stereo cameras were all 35mm. Using modern LF cameras, with a single lens board, as I mentioned in a previous post, does NOT have the geared precision to focus at very close distances, such as 15ft, 18ft, etc, which was intended purpose of this camera. This was all part of the learning process of the thread above, much was discovered through collaboration.... it even made me think and re think what I was doing... you may want to re read some of it.

> For what you want to do, concentric boxes will do for moving the lenses forwards and back and keeping the their axes normal to the film plane.

I have never seen any concentric box as effective as a helical mount, whereas the gearing is specifically designed for a given lenses focus requirements? I doubt any generic box can match this precision. And, the helical mount is not custom, it's readily available from the lens makers? That's all part of the beauty of the final design, i can live with the lens spacing issue.

> A limiting factor that may force you to wider inter-lens spacing than you want is the diameter of the lenses' shutters.

No, the Copal 0 shutters are 61mm wide. The helical mount was the limiting factor. And I am convinced there is nothing as elegant as this solution.

> But then, that's because you're thinking of lenses in big shutters.

Big shutters? Copal 0? They don't get much smaller, and even if they did, I have no need for smaller?

> Lenses that will do for you were sold in little ones too. I just measured the #00 Prontor Press my 47/5.6 SA is in, it is a little over 60 mm o/d.

Well, that is about the same o/d as Copal 0 shutters brand new, which are factory tested, which will help me acheive good shutter sync? So why go with older shutters?

> And if you start from a severely hacked up 4x5 Speed Graphic, you can probably avoid using lenses in shutters.

Focal plane shutters are very appealing in theory, but in reality, they would not be well suited here for several reasons:

1. They are simply not wide enough, as my exposure area is 138mm, bigger than 4x5.

2. They do not work well at fast shutter speeds, 1/500th.

3. Excessive vibration / motion reduces sharp focus, which is what all this camera is all about. If extreme sharpness was not an issue, I could buy a 1930's vintage MF stereo camera, such as a Rolleidoscope, which was Rolleis first camera ever produced. For their vintage, resolution is very good, but no match for todays superior optics. It is interesting these cameras still sell in the \$3 - \$4k range.

> Now, why did you rule out the old standard solutions to the problem?

I hope the above explains why......

Thanks Dan

GPS
2-Aug-2005, 16:05
Dan, if you have been looking at this thread with dismay and hilarity, you can just continue doing so. Working in a patent office in NY you can see about 200 types of these invention heroes a month... So far the best was the question about focusing on the surface or the middle(!) or the bottom layer of a film - with your naked eye it's just a piece of cake.. That made my day.

bglick
2-Aug-2005, 19:43
gps, it gives me pleasure knowing I am making you happy.

> Working in a patent office in NY you can see about 200 types of these invention heroes a month...

Invention hero? hmmmm, I needed a camera to complete a task, none existed, so I learned how best to build one. Hero, no. Amateur camera builder, well, OK.

> So far the best was the question about focusing on the surface or the middle(!) or the bottom layer of a film - with your naked eye it's just a piece of cake.. That made my day.

I beleive you were referring to this which i wrote above....

"Also, if you are perfectly focussed on the gg, this would represent the very front of the film plane...shouldn't the focus be at mid point of the film thickness?"

This made your day? That's strange. I posed this question to two camera designers since I wrote this. I have learned this issue is of major signficant to many camera builders. For starters, I was not talking about ones eye seeing the difference between focussing on the front of the film vs. the back of the film. I was asking, where is the etching of the gg aligned to, front, middle or back of the film. I have sinced learned the answer is the front of the film plane.

Without getting into tremendous detail, as it seems your looking for a laugh, not collaboration...but at very low f stops, with sharp lenses, sharp film, the plane of sharp focus (depth of focus) is .016 mm. One camera designer, probably another invention hero, had to have a shim system to adjust for the variance in manufacturers film thicknesses, so as the helical focus system would be accurate for each type of film used. Even Kieth Canham went to extremes keeping his film plane in 6x17 back within .0001" of the etching of his gg. So the thickness of the film and focus point is a consideration when using very sharp film, sharp lenses and very low fstops. It has nothing to do with seeing these differences with your eye, which we all know is not possible. For landscapes with lots of DOF, this is not an issue, but for copy work, or close focus, yes it is an issue. Bottom line, if you want the best results, everything should be accounted for. In LF camera systems that shoot at f32, this is not even an issue.

Hey, in the end, we both gained, you got a good laugh, while I gained some very useful knowledge from other Invention heros, you know those loosers, who actually produce useful products instead of badgering others who are learning ;-)

thanks for your input gps...

GPS
3-Aug-2005, 02:12
How pathetic! Now all that remains is to tell us how to focus with the helical focusing mount exactly at the uppermost layer (or the second or the third etc. -there is a lot of them) of the film's surface coating. Not to mention that if you are after this precision you have to follow it all the way around - on the septum, on the thickness of the film holders, on their inner film position, on the gg frame etc. etc. Long live invention dudes! 201 this month.

Kevin Kemner
3-Aug-2005, 08:49
Bill,

As one of those invention dudes out there, I've been giving some more thought to your project and would like to offer the following.

1) The double helical focusing for a stereo camera has an inherent flaw when considering close critical focus. It will be practically impossible to match the focus of the two helicals given the parameters that you have set.

2) A better solution would be to rethink the rack and pinion focus. Extremely fine control can be had with such a system. The fineness of focus is a result of the circumference of the finger knob and the gearing ratio. Mamiya once offered a solution to this for their TLR cameras by producing an oversized knob that greatly increased critical focusing. This approach would also allow the two lenses to be locked together in focus.

3) Perspectival control could be somewhat addressed by a shifting lensboard design similar to that of a Cambo or Alpa SW camera. The shifting plate is more stable than tilt and would not add the challenge of the stereo lenses equally maintaining focus when tilting.

4) A simple approach to the design of such a camera would have the shiftable lens front serve as the rigid frame of the camera with the film back be the moving element. One a smaller format it would be like taking an old Speed Graphic stripping it down to its frame and track. taking the film back off and replacing it with a lensboard with a shifting plate, kit bashing a finer focusing track and mounting the appropriate film back. It wouldn't be pretty but it would be stable and free of inherent flaws coming from dual focus and film plane movements. Oh, You would have to modify the bellows to address the reverse taper.

That's my 2 cents. You could even build this from scratch at a nominal cost and to be honest I think I'm going to put the 16x20 Pinhole on the shelf again and take a stab at this sucker. I'll try and post some CAD drawings next week.

There, invention 202!

GPS
3-Aug-2005, 09:44
Back to the "brillant idea" of the rack and pinion, Bill. Now, where has gone its shine? How will you solve the problem with the free play in the rack and pinion used to move on 0.3 mm? With enthusiasm, I suppose - as always when the knowledge is missing.

Dan Fromm
3-Aug-2005, 11:00
Not my yob, gps, but spring-loading might do the trick.

Cheers,

bglick
3-Aug-2005, 11:20
gps.... once again, thanks for your constructive input. you wrote...

"How pathetic! Now all that remains is to tell us how to focus with the helical focusing mount exactly at the uppermost layer (or the second or the third etc. -there is a lot of them) of the film's surface coating."

Well, you either have zero knowledge of what we are doing, or you can't comprehend what I am writing. Last time gps, I am was referring to gg and film position alignment! I thought I thoroughly explained this in last post...Yeah, its pathetic, but Lieca, Ziess, all have written articles on how intense this subject is. It's obvious you have much more knowledge then these design engineers....why dont' you go marching in their offices and tell them about how ZERO or Heros they really are. I mean lets make sure your points get through the real loosers, i.e. the biggest camera makers in the world!

Also, tell mamiya USA to stop wasting \$80k on laser systems to re align gg / film plane on Mamiya cameras and Toyo view cameras to .0001". How stupid is that? I sent my M7 in their once as this alignment fell out place from hard jolts, and a simple .001" made all the difference in the world, as the "eyes' of the people were now perfectly sharp. Lots of fools out there gps, all wasting time and money on this useless stuff......it's time you educate the world with your superior knowledge.

Anyway, if you have nothing constructive to offer, why not just read the posts, have a laugh and move on, in my opinion, you destroy the spirit of the forum. But, its a free world, so enjoy it, if this is what makes you happy.

Kevin, your points are VERY valid and I have considered them all. The single focus mechanism with a huge geared ratio advantage is ideal, however, I see one big trade off here. Since we are dealing with focus ranges which are so small, just a slight cock of the long lens board, (on the view camera arrangement) would mis align the left / right focus which I can not correct. Whereas with dual focus, I can check the gg (although not perfect, its the only cross check I will have in the field) and make sure each lens is focussed at the exact same point.) If the camera was not going to have lens rise / fall and tilt, then I would fully agree with the dual focus approach as every part will remain tight once aligned. On the handheld version of this camera, with no movements, this would be ideal, but I will already have the helical mounts in the lenses, so........ whats your read on my thoughts? Again thanks for constructive input.

Bill

GPS
3-Aug-2005, 11:38
Once again Bill, I'm overwhelmed by your constructive input. You did not answer the question how you will focus on the uppermost film layer but at least now I know that Mamiya has solved the problem for you. That's what I call an intelligent solution! So now it's just to make the box and...

GPS
3-Aug-2005, 15:41
You see Kevin, just a slight cock of the lens board could missalign the lens board while manually focusing the left and the right lens and checking visually the focus of both lenses will make sure that each lens is focused at the "exact same point". This guy thought of it all. What he doesn't know is that if anyone tries to focus several times the same picture he always ends with putting the focus slightly differently - thus the stereo will be really "stereo pronounced" with this clever solution. Time to call Mamiya again?

bglick
3-Aug-2005, 16:38
gps wrote.... What he doesn't know is that if anyone tries to focus several times the same picture he always ends with putting the focus slightly differently - thus the stereo will be really "stereo pronounced" with this clever solution. Time to call Mamiya again?

gps, this is TRUE. But its not a blanket statement. The degree at which people mis focus is highly dependent on several variables, such as fl of lens, focus distance of subjects, precision of focus rail, magnification of ground glass loupe.... with all these variables in your favor, you can do quite well at re focussing on the same subject. However, when all the variables are not in your favorable, granted, focus can be inconsistent.

However, the point you are NOT comprehending is..... regardless of the human factor here, are you suggesting it is therefore OK to let all the other controllable focus variables become sloppy, mis aligned and suffer from lack of precision since humans are not perfect at focus? The goal, I would think, (being a crazy loony tune inventor) is to tighten up all the controllable variables through proper design, machining, materials and thorough testing under the same circumstances the camera will be used for. Then, you have a camera whereas, hopefully, the human elelment is the biggest remaining variable to poor focus.

But, as you mentioned, I have been stupid from the start and this exercise provided nothing but humor for you.... so who knows, maybe your thinking is best.... since humans are not perfect, let the camera system be sloppy anyway, its all a waste of time and energy. (Lieca, Zeiss, Mamiay, etc. etc.??) If this makes you feel good, then you should build your cameras with your design philosphy. I won't knock it gps and would only wish you the very best of luck in your camera project. But is it OK I stick to my design philosophy, without you badgering me to death?

Thanks again for the constructive input.

GPS
3-Aug-2005, 17:41
You're welcome. "Since humans are not perfect at focus" how will you focus at the uppermost film layer? Mamiya ex machina?

Paul Fitzgerald
3-Aug-2005, 20:28
HI there,

How did this get personal???

Bill, this all sounds like a very long way around the track to justify that roll-film back, whatever.

The sliding box-in-box is call Rolleiflex and it works quite well AND simplifies construction. A third lens for reflex-focus also makes more sense for precision than on-again-off-again with the gg and film holder. ZERO tolerence for the boxes can be had by rebating the ends of each box for a delrin or teflon glide rail around the perimeter. This would also adjust for thermal expansion. Adapting the eye piece/view screen from any medium camera should not be hard.

It would be faster, easier and cheaper to just buy Gilde's camera.

Roll film ain't large format.

bglick
3-Aug-2005, 21:14
Hi Paul

> How did this get personal???

Read above ......

> Bill, this all sounds like a very long way around the track to justify that roll-film back, whatever.

The 6x17 back is a means to an end.... no other way to get two images this close on film...

> The sliding box-in-box is call Rolleiflex and it works quite well AND simplifies construction.

If two Rolleiflex's slammed together would produce the inter lens spacing I desire, this thread would have never existed. They are a long way off from what I am trying to accomplish, otherwise, I would have surely used them. There is few of the old mamiya reflex cameras that come close, but optics are aged vs. todays super sharp optics, which is the other motivating drive behind this camera.

> It would be faster, easier and cheaper to just buy Gilde's camera.

Faster? Gilde is about 9 months delivery? Cheaper? \$18k for same set up, less versatility...mine will be less than half this? Easier - Yep, but the camera does not have the lenses spacing I desire, and once again, this was the big part of the reason I did not twin two cameras. I have wired my two Mamiya 7II's together and have near perfect sync, right up to 1/500th....but lens spacing, even bottomed together, with no bracket in between cameras is 90mm. Side by side, to get the full horizontal image the lens spacing is 165mm. This is considered hyperstereo, not what I need for this camera.

> Roll film ain't large format.

Yep, that much I know.... but the camera pieces I am using sure are. Maybe I am missing your point here? I considered making this caamera with two 45 degree mirros behind the lenses and using 4x5 film, but there is little to no advantage of such, as the DOF losses at higher f stops degrade the sharpness at the point of exact focus....MF is the ideal format for the best stereo.

gps, last response.....

> Since humans are not perfect at focus" how will you focus at the uppermost film layer

You align the etched portion of the gg to the front of the film layer, then a human will do the best possible to focus on etched portion of gg. Is there some other focus method ever invented in photography that I am not aware of? Human looks through some focus screen, adjust focus accordingly, and hopes his system is aligned well enough to produce best possible results. Pretty simple. All this was written above.

GPS
4-Aug-2005, 01:08
"You align the etched portion of the gg to the front of the film layer" ....hmm. But the question was, if you still didn't get it, how? Calling to Mamiya? "Pretty simple. All this was written above" Only dudes give this kind of answers.

GPS
4-Aug-2005, 01:12
To those interested - Bill has found the way how to align a gg to the uppermost film layer! Pretty simple - all was written above. The uppermost layer is a UV layer not the silver halid one - will Bill go one step under? A piece of cake, just some mikrons deeper...

Struan Gray
4-Aug-2005, 01:32
There seem to be two problems here. One, how to shim the film plane and ground glass so that it is at the right position and orientation; and two, how to focus two lenses at once while maintaining their correct positions with respect to each other and the film plane.

The solution to the first problem has been solved to photographic accuracy by every modern camera maker. The simplest is to make your parts to aerospace or scientific accuracy, and out of materials that won't warp. 0.1 mm is easy and repeatable with the right tools. Failing that, shims or adjustment screws (use three for stability) will do the job. Use micrometers if you can afford the bulk, as in the much-maligned but technically sound gigipixel camera (www.gigapxl.org). Otherwise, the optics suppliers can sell you lovely reducing screws: for an example look under "micrometers and nudgers" or "screw sets" at www.newfocus.com.

For manual focus one option is to take the gigapixel route and make a table of micrometer settings for different distances. Or you can focus one screw by eye, and keep a table of settings for the other micrometers that will ensure parallelism. This is a royal pain, but accurate to a micron or so given the right micrometers or lead screws. Another option - better in my view - would be a single helical or lead screw arrangement large enough to encompass both lenses. A lightweight version of my old vacuum translator (www.thermionics.com/ect.htm) would do the trick, but if you don't mind a bit of weight a single lead screw with twin guide rails will do the job (www.thermionics.com/fbt.htm).

GPS
4-Aug-2005, 06:40
Struan, Bill has already found the way how to focus to the uppermost film layer (he doesn't say how but never mind, talk is cheap). While in Mamiya they need "80 k laser system" he will, no doubt, do without... 0.1mm precision you try to sell him doesn't work for the uppermost layer. Neither will an adjustment screw! If you think it will do, then as Bill says go to Mamiya and tell them "to stop waisting 80 k on a laser system"... etc. etc. The technical ignorance and naivity is obvious throughout his cheap talk threads.

Struan Gray
4-Aug-2005, 07:37
Sorry gps, I'm not interested in your Kill Bill Klub. Even with the decoder ring.

GPS
4-Aug-2005, 07:52
Sorry Struan, no need for a Kill Bill Klub on my side either. This guy, calling himself a "pro photographer, engineer" started the thread with the incredibly dudy question about focusing LF lens by moving the front and the back lens group apart. Then he goes on by saying to the people that he now constructs a home made camera capable of focusing on the uppermost layer of a film (after asking where to focus - the surface, the mid layer or the base of the film?). No need for a Kill Klub. Just trying to say to him and to those like you that focusing on the uppermost film layer with a home made camera is like trying to go to Mars on your bicycle. Do you mind?

bglick
4-Aug-2005, 10:59
Struan, it's always a pleasure to have your input. You have been VERY helpful in these forums, (specially to me) thanks for your continued participation.....

> One, how to shim the film plane and ground glass so that it is at the right position and orientation; The solution to the first problem has been solved to photographic accuracy by every modern camera maker. The simplest is to make your parts to aerospace or scientific accuracy, and out of materials that won't warp. 0.1 mm is easy and repeatable with the right tools.

Agreed.... Canham uses a aerospace defense contractor to machine all his metal parts, they are machined with tolernaces in +/- .0001". Hence why I feel comfortable his gg and film plane alignment is as precise as I can acheive for a home brew camera. So I felt this part was - a good fit.

> For manual focus one option is to take the gigapixel route and make a table of micrometer settings for different distances.

I read about the gigapixel camera.... a few big differences, he uses much longer lens, 200mm+ and shoots at very high vs. f stops vs. what I plan to do.... but he sure did overcome many obstacles with that beast. And talk about a home brew camera...anyone who has not visited their site should see just how the application of knowledge and machinery comes together in camera systems.

> Or you can focus one screw by eye, and keep a table of settings for the other micrometers that will ensure parallelism. This is a royal pain, but accurate to a micron or so given the right micrometers or lead screws. Another option - better in my view - would be a single helical or lead screw arrangement large enough to encompass both lenses. A lightweight version of my old vacuum translator (www.thermionics.com/ect.htm) would do the trick, but if you don't mind a bit of weight a single lead screw with twin guide rails will do the job (www.thermionics.com/fbt.htm).

A single helical would be ideal, theoreticaly .... but in the real world, I don't know if I could overcome the additional obstacles presented by the single helical, such as.... where would the helical be placed as their is NO room between the lenses... therefore it would be off center axis, not desireable. How would this long lens board telescope in/out paralell on both sides? Yes, there is fixes for that as you mention, but very labor intensives for each shot. Also, then I would need a different helical for differnet lens fl's?

On the other hand, it seems the factory made helicals made for the exact lenses you are using.... they have mastered the focus ratio needed for each lens.... with two helicals, it adds additional obstacles, but if the rig is checked well and proper calibrated, to assure each is focussed at 15 ft, at the 15 ft helical marking, then I will be reduced to using a high-end rangefinder type camera (when not using the gg) So testing and calibration is my best tool, to be assured, I am focussing where the markings suggest.... i.e. film under a loupe....shoot at target, and slightest movements behind and front of target.....

In the end, in my opinion after really thinking this through...... I think what is saving me on this camera is...... f11. If this camera system needed to shoot at f2, well, I doubt I could overcome the "depth of focus' issues at the film plane with two lenses...this is where you need a super precision fixed box, 35mm type.....but f11 allows some slack, but not enough for conventional LF gear to focus accurately...so the factory helicals in my case are the bridge between precision camera system and a LF system, which affords the benefits of movements and gg focussing / composition.

I am thinking out loud Struan, please comment - is my assessment above valid in your opinion?

As I mentioned earlier, the lens board will be used on two different versions of this camera, one is the gg focus version for methodical landscape shooting and the other is the handhold version, whereas the later needs to be tested very well, as the helical markings need to be trusted to prevent slightly mis aligned focus between the two images, causing retial rivalry when viewing the slides which are magnified 4x in a stereo viewer. Stereo adds MANY more obstacles vs. 2d.

As always Struan, I appreciate your input.

Bill

GPS
4-Aug-2005, 12:03
"I think what is saving me on this camera is..... f 11" (Bill). How come? Something going to be screwed up? But then - where is the fine focus on the uppermost layer of the film? What? There is no road to ride a bicycle to Mars? Then I need a constructive input. No problem, it's just to take the bicycle on my shoulders!

GPS
4-Aug-2005, 12:09
"Stereo adds MANY more obstacles vs. 2d." (Bill). Sure, twice as many.

GPS
4-Aug-2005, 12:12
As always, Bill, I appreciate your input.

GPS
4-Aug-2005, 12:27
Ok Bill, I'll cut you a slack. A few years ago there was a guy (in comparison with him you're just a weak disciple) on this forum constructing a LF camera much better than everything on the market. He went so far with his cheap talk so as to promise to those interested in his invention a special "friendly" price (after having the camera patented, of course). Believe it or not there were those who asked for it! When it was the moment to deliver instead of to talk, he disappeared from the forum - only to appear much later, saying he's got other things to do for now (moving to a new house or so, I don't remember). No new camera, no patent, no special price. Be better! Cheers!

Struan Gray
5-Aug-2005, 02:40
Bill, I recognise your dilemma. It reminds me of the time I was a newby student taking the drawings of my first gadget to the instrument workshop to get it made. The kindly chief technician sat down with me, complimented me on the fact that I could actually produce a decent scaled drawing, and then took the design to the cleaners. The problem was not that my gadget wouldn't work, rather that I wasn't clear in my own mind about how I wanted to use it and how I wanted them to build it. Since then, I've had the luck to work with several similar mentors who had the happy knack of making what I wanted rather than what I asked for. At present you seem to be thinking out loud in a vast sea of options, which is fine but they can't be distilled down to a final design until you start narrowing your options.

The devices I pointed to are designs suitable for precisely moving a fixed lensboard with both lenses mounted on it. If you are going to the trouble and expense of getting matched lenses this is the route to take: if you put them in seperate helical focussers there's no real need to match them. Wide bore helical tubes that would do the job are available, or can be made, but something that encompasses two 47 mm LF/MF lenses and their shutters is going to be bulky, heavy and expensive. A final simpler option would be an adjustable scissor strut like those found on countless focussing folder cameras. Grimes or any other instrument-quality machinist can make a version of any of these for you if that's what you really want and need.

As others have said, without an encompassing helical tube you are going to need a light baffle of some sort. A single-pleat bellows or sliding boxes are the classic solutions, but for really short travels you could just use a fat O-ring.

On the other hand, if you can live with f11 or smaller, and only focussing down to 20 feet, there is no need for a focussing device at all. Assuming the images will be 6x8, the conventional hyperfocal distance for a 47 mm lens at f11 is about 4 m, or just over 13 feet. Even at f5.6 you're only at 26 feet. On the handheld you could just live with zone focus (I would set the registration to the hyperfocal distance for f5.6, then stopping down just makes things better). On the full-blown camera you could use a geared rear focus to fine tune things or to focus closer. If you must have really fine focus, you could gear down the rack-and-pinion drive on the back, or incorporate a small linar drive from an optics catalogue.

bglick
5-Aug-2005, 04:54
Struan

> Since then, I've had the luck to work with several similar mentors who had the happy knack of making what I wanted rather than what I asked for. At present you seem to be thinking out loud in a vast sea of options, which is fine but they can't be distilled down to a final design until you start narrowing your options.

I can certainly understand how you feel that way from this massive wandering thread, but I have narrowed it down, to exactly what I want. Admittedly, this was occuring through the thread, and while speaking to camera builders, lens makers, etc.

> The devices I pointed to are designs suitable for precisely moving a fixed lensboard with both lenses mounted on it. If you are going to the trouble and expense of getting matched lenses this is the route to take:

I did look at the links.... and although I certainly don't disagree with your position, it comes to down to practicality of finding someone to assemble these pieces, the weight and size, the method to utilize these fine focussers.... how much loss would be in the telescoping lens board? Where is the physical space to do all this in a tight compact field camera? Get my drift here? If this was a studio camera whereas were designing them to be sold in volume, then I am sure an better system can be had....

> if you put them in seperate helical focussers there's no real need to match them.

I learned this the hard way.... lenses vary tremendously in fl's , even in the same batch. If they were not paired, then I would be allowing a potential issue creep in, which I have no way of testing, hence why I have the factory laser test the lenses and pair them. Once again, try to eliminate all the controlable obstacles.

> Wide bore helical tubes that would do the job are available, or can be made, but something that encompasses two 47 mm LF/MF lenses and their shutters is going to be bulky, heavy and expensive.

Yep, I have to stick to Copal 0 shutters, and sync them externally.... not easy, but doable, I have had success at this in the past.

> A final simpler option would be an adjustable scissor strut like those found on countless focussing folder cameras. Grimes or any other instrument-quality machinist can make a version of any of these for you if that's what you really want and need.

Yep, this was my first concept.... till I realized how difficult it is to keep scissors straight on 6" wide lens board. With single lens, with optical axis in middle of board, this is perfect, but with two lenses, both off axis from board center, all slight mis alignments are exaggerated. With the factory mounted helicals, I can always see them on the gg and control them seperatly with a very acceptable geared ratio. I still can't comprehend a better system for this application.

> On the other hand, if you can live with f11 or smaller, and only focussing down to 20 feet, there is no need for a focussing device at all. Assuming the images will be 6x8, the conventional hyperfocal distance for a 47 mm lens at f11 is about 4 m, or just over 13 feet. Even at f5.6 you're only at 26 feet. On the handheld you could just live with zone focus

I need the ability to focus at certain distances, albeit a tight range, say 9 ft to 18 ft..... but quite often I need the sharp focus directly on the subject, such as in portrait type pix. At f11, and no higher, resolution at the focal point is extremely sharp vs. the near and far.... hence why I worked so hard for focus in this camera. Its also why I called this rig a bit of a hybrid.

> (I would set the registration to the hyperfocal distance for f5.6, then stopping down just makes things better). On the full-blown camera you could use a geared rear focus to fine tune things or to focus closer. If you must have really fine focus, you could gear down the rack-and-pinion drive on the back, or incorporate a small linar drive from an optics catalogue.

Rack and pinion drive on view cameras is no where near accurate enough for focus this fine. I tested it on a few precision view cameras. Again... even if I found a precision sinar with better fine focus, I still can't imagine it outperforming the helical mounts from the lens maker? I guess I am confused why you are bucking against the helicals... I really beleive the other methods would work also, but trying to find someone to do all this work, testing it, reliability for years to come, the size and weight of the camera, etc. ..... in the end, it seems the helicals keep winning, they are time tested for this exact task.... if you machine the lens board mount so its square to film plane and at proper distance, everything should consistently work.

Bill

Dan Fromm
5-Aug-2005, 05:02
Bill, take a look at how microscope focusing mechanisms work. They give finer control than you need, and millions -- literally -- have been made.

bglick
5-Aug-2005, 05:12
Dan... I agree microscope focus mechanism are very high precision, and more than I need. And I could easily hack one up for its gear system.... but as I mentioned to Struans post.... now, find away to put this on a lens board assembly whereas there is only 47mm of space to work, two lenses, etc. Now the geared mechanism becomes the easy part..... now the customization is..... what exactly is this geared system moving? So you gain one off-the-shelf item only forcing you to custom make another part to utilize its function. This once again is the beauty of the helical, its centered around the lens and removes that huge variable....., which makers of folder cameras never had to contend with, as they had the lens axis in the middle of the board they were controlling....

Struan Gray
5-Aug-2005, 05:36
Bill, I'm avoiding individual helicals because you said you wanted to avoid then if you could. They'll increase the seperation of your lenses and the amount of fiddling you have to do before you take a photo. I and others are just pointing out that if you want to explore other options, they are out there. As Dan (and I) said, there are better rack and pinion focussing tracks available for \$100 or so if you need them, or you can use a micrometer or a differential screw drive like these (http://www.optosigma.com/miva/merchant.mv?Screen=PROD&Store_Code=OS&Product_Code=pg369-370&Category_Code=Accessories), pushing on a linear bearing if you want micron accuracy.

To me, it makes sense to shim the lenses once and use a single translation mechanism to move them together as a unit. I agree that a helical drive is simple, intuitive and robust. If you have the budget, go for a single wide-radius helical with a bore large enough for both lenses. You'll get guaranteed synchronised focussing, the gearing will be even better than on a helical for a single lens, and you halve the time spent looking at ground glasses. Finally, you can use the same focusser for 47 mm and 80 mm lenses, reducing overall system compexity. I'm sure Grimes could make such a thing, and if not optics houses like Melles Griot or Edmund Optics certainly can.

Finally, if you do go with a single focusser, consider a single shutter like the Sinar shutter or a big 'ole alphax. That would let you use lenses in barrel (no problems getting them close together) and ensure synchronisation of picture taking and flash. The only downside would be a reduced max shutter speed.

Struan Gray
5-Aug-2005, 06:07
I just can't help myself.

It ocurred to me that you might not understand what I mean by a 'large radius helical'.

Imagine a 150 mm square lensboard with a 140 mm diameter hole in it, surrounded by a cylindrical lip that is threaded with a male 140mm x 1mm pitch thread.

Now screw onto that a circular collar with a matching female 140 mm x 1 mm thread on the rear, and a female 140 mm x 1 mm left hand thread on the front.

Now screw onto that a custom board with a 140 mm x 1 mm left hand male threaded lip on the rear and a regular lensboard attachment on the front.

In one or more corners, mount a pin that is fixed to the rear board and passes through a tight hole on the front - this stops the front and rear boards from rotating with respect to each other.

Mount your lenses side-by-side on a regular lensboard, and attach them to the front of the contraption.

Rotating the collar moves the boards 2 mm apart for each turn. Add a focus handle to the collar, and use only one corner pin so you get one whole revolutoin, and you have a device with enough motion to focus the 47 mm lenses to 3 feet, and the 80 mm lensee to 10 feet.

You'll need a spacer to increase the infinity distance for the 80 mm lenses (make it part of their lensboard). You may need to include a concentric light baffle inside the tube.

Struan Gray
5-Aug-2005, 06:08
OK, OK, make the hole 138 mm. Or 136 for safety's sake.

bglick
5-Aug-2005, 18:40
Struan, thanks for your presistence in helping.....

> Bill, I'm avoiding individual helicals because you said you wanted to avoid then if you could. They'll increase the seperation of your lenses and the amount of fiddling you have to do before you take a photo.

Ahh.... OK, I see.... well, in the end, the compromise in center lens spacing was small after hacking up the factory helicals a bit......but something I still want to consider as center lens spacing is critical....

> I and others are just pointing out that if you want to explore other options, they are out there. As Dan (and I) said, there are better rack and pinion focussing tracks available for \$100 or so if you need them, or you can use a micrometer or a differential screw drive like these, pushing on a linear bearing if you want micron accuracy.

I fully comprehend this....

> To me, it makes sense to shim the lenses once and use a single translation mechanism to move them together as a unit.

And I agree with this too, always have.....more below....

> It ocurred to me that you might not understand what I mean by a 'large radius helical'.

Yep, Struan, I did miss your point... I just plain did not visualize a huge helical going around the two lenses.....sheeeesh, that is just plain brilliant! You still amaze me.... I will discuss this with others to evaluate pros and cons.. and I may devise such a rig, as single focus mechanism is surely desireable...... but, there is one issue that may prevent me from going forward with it. With the dual camera system I am making....i.e. meaning the lens boards will switch between handheld version and veiw camera version.... I would have to maintain the same lens board for both...... it's possible, this needs to be explored...and also the ability to use rise and tilt on the view camera arrangement may cause some issues here with massive helical, unless its actualy part of the lens board....

> Finally, if you do go with a single focusser, consider a single shutter like the Sinar shutter or a big 'ole alphax. That would let you use lenses in barrel (no problems getting them close together) and ensure synchronisation of picture taking and flash. The only downside would be a reduced max shutter speed.

Ahhh, once again, you nudged me in the right direction..... Although no modern focal plane shutter will work for an image this big, sinar does make a 6x4.5 focal plane shutter. Now, I can affix a pair of these to a rear lens board, and it will mount on and off with each lensboard, so they can be used for other lenses. Not sure I can get the electronics wired externaly, or just how much electronics is required from their digital cameras.... but if this works, it may be fabulous, as I now have shutters up to 1/2000 th, perfect for outdoor shooting of fast moving subjects! It is much easier to sync electronic driven shutters vs. full mechanical shutters. The old shutters, even 4x5 are not big enough, although one would fit nicely behind the lenses...

Which made me think.....any leads on a large focal plane shutter that can cover about 120mm, 5"? The old graflock versions of these are just way too clunky, although some were fast, 1/1000 th on 4x5 format, which is good enough. Huge benefit to one single shutter, perfect sync is guarantted, but I must be careful not to add vibration which will degrade image quality quickly.... I can't seem to find any at the link you provided....any other sources? Does any place custom make these?

With any luck, I can now have one single large helical foccuser (vs. 4 hellical mounts) so all lenses can be purchased in barrel, then a single fl shutter behind the lens. I must admit, if this pans out, it will be a better system than my previous system.....

THANKS STRUAN! Keep it coming! :-)
Bill

bglick
5-Aug-2005, 20:52
Ahhh... LCD shutters.... it seems they would be perfect.... even if they cost me 1.5 stops of light, but perfect sync.... I may have to start a new topic to see if there is any experience users of such.

Kevin Kemner
5-Aug-2005, 23:47
Wow Bill,

This thread has been all over the place. I reread through the threads above and then took some time to do a little research and want to throw one last set of thoughts out for you. First off, I work with engineers and industrial designers all the time and will try to approach your problem as they would.

So lets get down to basics. You are trying to build a 6x17 medium format stereo camera based using either 47mm or 65mm focal lengths. Your primary need for this project is an as yet unstated project with focal distances between 15 and 20 feet. You would like to have a second use of the lens assembly for a hand held hyperfocal landscape stereo camera. You have expressed concerns about film alignment precision and the ability for very small focus movements. You intend to use ground glass focusing.

Okay, lets look at limiting factors. We do not know how you intend to reproduce your images. As you are looking at stereo photography we could assume that you will be using a viewer of some kind to look at these images. The images will probably be small in that case. As we don't really know how you intend to reproduce these images lets wait for this information.

We do know that you intend ground glass focusing. At the format size you are using, ground glass focusing is incapable of matching the resolving power of the lenses. This is a weak point in your camera if you are obsessed with precision. One reason that MF view cameras are rare is the inherent inability of ground glass focusing to match the rangefinding or split screen focus systems of current medium format cameras.

Ground Glass and film plane calibration. Its pretty common that ground glass backs and film holders are not in the same focal plane. This is an easy fix and all you need is some precision brass shim, a 12" bar of brass flat stock and a plunge micrometer. The fix can be done in less than a day and will show significant improvement in your images. As for consistant lens axis to film plane alignment, if you intend to use movements the best you can hope for is close alignment rather than perfect alignment. It is practically impossible to have a view camera with perfectly aligned axis.

Okay, what works in your favor? Surprisingly, DOF and Hyperfocal distances are all in your favor. I ran some dof calcs for your camera with the lenses at f5.6 and as near as I can tell your camera should have a dof range of at least 10' through 25' for your project. It gets better at f11. In fact the dof for this camera would be so good that you good be focused a good 2' off of your subject and it still would be in sharp focus at f5.6. So, where does that leave you?

Much better off than you think. For your close focus project all you need is a monorail camera capable of mounting your film back, of using a bag bellows, and with a large enough lensboard to mount your two lenses. Its not glam but it could be cheap.

Anyway, give up on the complicated solutions and go simple and straightfoward. Find a camera that meets your needs, get a couple lensboards drilled out and get to work.

Now if you want a field camera check out these two sites.

http://phsne.org/stereocameras/STEREOFORMATS.HTM

http://www.ghouse.com/daniel/stereoscopy/equipment/

Sorry they're not links.

Kevin

bglick
6-Aug-2005, 02:56
Kevin.....

> This thread has been all over the place.

yep, this is what happens when great minds collaborate! More than one way to skin a cat!

> So lets get down to basics. You are trying to build a 6x17 medium format stereo camera based using either 47mm or 65mm focal lengths. Your primary need for this project is an as yet unstated project with focal distances between 15 and 20 feet.

Focus distances can range based on the fl lenses being used. The two fl's you mentioned are my basic two, next would be an 80mm fl.....so maybe 3 total...but this does not change the focus issue....

> You would like to have a second use of the lens assembly for a hand held hyperfocal landscape stereo camera.

Well, hyperfocal was suggested by many, but not by me.... more importantly, with out making things even more involved in this thread... I have all the DOF issues, cc, magnfication of viewer, losses in reproduction, etc. worked out and proven. In the end, I need these lenses so focus at distances between 9 - 18 ft, as a basic range, maybe up to 27 ft with the 80mm lenses, but not that signficant.... as it's hard to change focus from 12 to 15 ft with any of my proposed lenses without these radical procedures.

> You have expressed concerns about film alignment precision and the ability for very small focus movements. You intend to use ground glass focusing.

The handheld version will be..... set lenses at desired focus distance, look through top mounted viewfinder and fire away, I must trust the focus scales, through much prior testing. As one of the important issues is focussing on a subject, like people, at the distance they are at....so if I have someone at 15 ft, I need to be focussed close to 15 ft, not 12 ft and not 18 ft.... and since the change in this focus range, +/- 3ft represents less than .1mm of lens movement to and fro the film plane...... that was the basis of all this theorizing on the best way to accomplish such precision movements on a non conventional camera.

> Okay, lets look at limiting factors. We do not know how you intend to reproduce your images. As you are looking at stereo photography we could assume that you will be using a viewer of some kind to look at these images. The images will probably be small in that case. As we don't really know how you intend to reproduce these images lets wait for this information.

yep, images are put in mounts, viewed in a viewer with 70mm fl achromat doublet lenses.... but I did not want to get into this, as I was focussing on the subject of camera focus :-) Images will be reproduced also, but it's irrelevant to making a camera focus on intended target accurately.

> We do know that you intend ground glass focusing. At the format size you are using, ground glass focusing is incapable of matching the resolving power of the lenses.

Very true.... but that does not prevent one from focussing accurately on a target.

> This is a weak point in your camera if you are obsessed with precision.

In theory this is a weakpoint in EVERY camera system. No viewfinder can resolve what film / lens can resolve, however, with a 3x loupe on the gg and very tight helical like Struan suggested will allow me to hone in right on the subject...(assuming I have the gg / film plane licked) so maybe if the subject is at 15 ft, I may get it to 14.5 to 15.5 ft.... which is much better than 12 - 18 ft..... and it takes all this focus precision to make this improvment.... I am not being crazy here.... but just getting close to the focus distance you want takes tremendous precision of lens to film position...then as you state, there is still slop in the system.....

> One reason that MF view cameras are rare is the inherent inability of ground glass focusing to match the rangefinding or split screen focus systems of current medium format cameras.

True, and what seperates view cameras from these MF cameras is precision focus at close distances.... not to mention slop in the standards, etc. So much of this will be eliminated in my view camera set up, and definetly in my hand-held arrangement as everything will be fixed, just like an Alpa. But not as pretty of course. Anyway, thats why this camera is more of hybrid, leaning more towards MF vs. LF.

> Ground Glass and film plane calibration. Its pretty common that ground glass backs and film holders are not in the same focal plane. This is an easy fix and all you need is some precision brass shim, a 12" bar of brass flat stock and a plunge micrometer. The fix can be done in less than a day and will show significant improvement in your images.

Kieth Canham tested his 6x17 back in his 5x7 back for quite some time..... he claims everything is within .0001" and set up for 120 film thickness and characterstics. So this was a big break for me, the fact the roll film back maker and the 5x7 back maker perfected this alignment, hence I do not have to re invent this portion of the camera.

> As for consistant lens axis to film plane alignment, if you intend to use movements the best you can hope for is close alignment rather than perfect alignment. It is practically impossible to have a view camera with perfectly aligned axis.

Agreed..... this is the price I will pay for movements, when I use them.... which won't be often, but I like to leave the option open. Hence why I want the fixed hand-held system also, a box with a helical.... no standards.....

> Okay, what works in your favor? Surprisingly, DOF and Hyperfocal distances are all in your favor. I ran some dof calcs for your camera with the lenses at f5.6 and as near as I can tell your camera should have a dof range of at least 10' through 25' for your project.

yep, however, I quite often need infinity in the scene.... just for your information...the way I do stereo calculations is to start with the min acceptable lpmm or cc which is acceptable when viewing the slides (obviously this is the near and far subjects) then I reverse engineer from there, repro losses, then viewer magnification, then f stop, etc..... now all this must fall within acceptable stereo DOF limits, so the images can be enjoyed in the viewer without forcing someone to do extreme convergence (cross eyed) which would ruin the entire experience.... anyway, this sets the entire near / far allowable conditions for the lens seperation and fl I am shooting with. Then I must make the lens focus at a distance to which will allow acceptable lpmm at my near and fars to meet min. criteria, with f stop being the only other variable I have to play with.....however, in this case, the digitars are optimized at f11, which is my limiting factor, (as 5.6 is to limiting in DOF and f16 causes too much diffraction at the point of exact focus) so focus is the ONLY real variable I have left to meet my crietera... and just trying to focus at 15 ft and missing by 3 ft, (.05mm on focus rail), and due to all the other eroding factors, its quite easy to fail at what seems so simple .....and missing focus point will completely throw off my resolution values I am trying to acheive.

However, the other more critical reason for sharp focus is when I shoot subjects like models, whereas I want there eyes in the sharpest focus, so now i am trying to hold +/- 3 inches. Surely this will be a challenge, even in the box camera version (hand held) with no standards, hence all the trouble I am going through to make this unique camera.

> It gets better at f11. In fact the dof for this camera would be so good that you good be focused a good 2' off of your subject and it still would be in sharp focus at f5.6.

You are not using the same lpmm minimums I am using..... 5.6 is not possible...... I can fill you in on the details of this, but it will make this post wander forever more.... I have been shooting MF stereo for 6 years and have seen all this first hand and know my exact goals....but I do appreciate you thinking this through like you have. I know what my two obstacles are to making this camera successful..... 1) abiltiy to focus at intended distance and 2) near perfect lens sync!

> Much better off than you think. For your close focus project all you need is a monorail camera capable of mounting your film back, of using a bag bellows, and with a large enough lensboard to mount your two lenses. Its not glam but it could be cheap.

Even in your example of missing focus by 2ft, it's hard to make a view camera focus to be within this range of your intended target as there is NOT enough precision to move the standard in .05mm increments (which matches up to your mis focus values) ...it's a crap shoot.... I shoot LF also, 4x5 and 8x10, and these are issues I never bothered with....

> Anyway, give up on the complicated solutions and go simple and straightfoward. Find a camera that meets your needs, get a couple lensboards drilled out and get to work.

If it was this simple, I would have done this long ago.... its not that simple for the precision I am trying to achieve, i.e. the on film resolution at the near and far, as well as the ability to focus within inches of my intended target. If you re read my post above about the precision on the fine focus knob of my Toyo VX125, you will understand why it will never happen.... and the VX is geared quite precise vs. most view cameras...maybe a studio camera would be geared a bit better, but this camera is for field use and needs to be handheld, so those cameras are not an option.

> Now if you want a field camera check out these two sites.

I have seen most of these antique cameras... in their day, they were wonderful.....but to take advantage of todays super high resolving films and very high resolving digital lenses.... precision is the order of the day...and if all i wanted was mediocre results, trust me, I would by a 1970's vintage Russion Sputnick MF stereo camera, they made 35,000 of them..... and for their nominal value, \$500, and with some tweaking, they can do quite a good job within certain shooting limitations.... but just like anything else, when you try to squeeze the last bit juice from something, the complications increase exponentially, not linearly, like we all hope for.....

A this point, I think Struan "monster helical" approach is most ideal for the focus issue...I will work to make this a reality.... if not, I feel like the factory mounted helicals is the next best option. If I surrender the ability to allow movements, then the Monsta Helical on a fixed box is best...it still allows for gg composition if I desire...I just loose rise and tilt capability, hence why I was trying to make the lens boards work on both....

BTW, the way I know the distance of a subject is with a laser rangefinder... i.e when not using gg focussing..... hence why focus scales will be needed.

At this point, I am trying to wrap up the lens sync issue, and the LCD shutters have me very excited.... as prior to this, I was ready to settle for mechanical sync, which is not easy at 1/125 +. I started another post on this. The application of these shutters may also dictate the final design of the camera....i.e. box version only, or box + view camera.... Its my bet, if these LCD shutters work out, they will be located behind the lenses...and this may cause problems with tilt as a 47 mm lens is already slammed against the gg.....

At this point, if could build in a little rise into the monster helical, I would forgo the tilt and settle on the box version.

Bill

Paul Fitzgerald
6-Aug-2005, 09:32
Hi there,

Kevin is right, ground glass would be too coarse for focusing, it would need optically flat plain glass with fine etched crosses to focus on, and then back&forth thru the loupe for parallax focus like ultra-fine copy work. Maybe B&L still makes glass like this, maybe AR coated.

Film flatness will not be held to such tight specs without a ceramic vacuum back like RTSIII, maybe Kyocera can make one up. Spelled major \$\$\$, they work down to a 1/4 wavelength of light tolerance.

What LPMM can human eyes resolve??? Overkill!!!

This has been happy high school bench racing, good fun.

bglick
6-Aug-2005, 11:39
Thanks for your input Paul

> Kevin is right, ground glass would be too coarse for focusing,

The only objective is to use the gg to find, and hold the sharpest focus plane....its not to have the focus plane record 60 lpmm..... this is no different than anyother eye/hand focus system.

> Film flatness will not be held to such tight specs without a ceramic vacuum back like RTSIII, maybe Kyocera can make one up. Spelled major \$\$\$, they work down to a 1/4 wavelength of light tolerance.

At f11, film flatness is should be fine.... The film can buckle up to .11 mm in either direction from the plane of sharp focus, and still maintain about 10 lpmm at the point of sharp focus, after magnfication in the viewer. This is why I commented many times, f11 is my savoir for this hybrid. But I do agree with your comment if I was shooting at f4. The 6x17 back was designed with a spring loaded roll holder to keep the film taught at all times, adding to flatness.

> What LPMM can human eyes resolve???

Avg human eyes can resolve 2 - 3 lp/mm. This is the min. I am shooting for my near / fars after magnificaion in the viewer..... so....

> Overkill!!!

Not sure what you are basing this on.... but my goal is to accomodate avg human vision? Overkill?

If your interested.... Ctein did a test where he showed people the same photograhic image, one at 5 lpmm, 10 lpmm, 15 lpmm, ...... through 30 lpmm. He asked the test subjects to place the prints in order from least sharp to sharpest. With almost uncanny accuracy, every test subjet placed the 6 prints in their proper order. My tests with the stereo viewers also supported his tests. The conclusion he made, which I agree with, even though humans can not resolve more than 3 lpmm on average, they can clearly discern and appreciate the differences in images. Unfortunately, due to large DOF requirements, I am shooting for only the min. resolution at the near and far and maybe 3x this at the point of exact focus.... but when I have subjects which require little DOF, I would like to offer my audience more resolution at the plane of sharp focus.

> This has been happy high school bench racing, good fun.

For me, its been serious discussion.....soon I lay out enough cash to buy a nice car to build this contraption. When in high school, I would not have the confidence to build such.....

Kevin Kemner
6-Aug-2005, 14:57
Okay Bill,

This is my last pass at this. I ran the lens to film distances for your focal and subject lengths and like you found that the lens moves in fractions of a millimeter as your subject distance shifts from 15' - 18'. However, as you are aware, your shorter lens will have very wide depths of field and even if you use circles of confusion appropriate to 35mm film cameras you will not see any difference in your focus over a range of 10' or so. Setting your f stop at 11 practically guarantees that focusing within 20' of the subject will provide an in focus scene throughout. When you start to get into your longer focal lengths it actually gets easier because you can see more clearly where your actual focus is.

In my opinion Bill, your trying for standards that are highly exaggerated for the equipment you will use and the possible results. As for how you intend to view these, using a hand held viewer, the lack of magnification will further simplify your task.

I appreciate your sincerity in this project of yours. What I suggest may be most helpful to you would be to contact someone like SK Grimes and see if they would be willing to help you out. My thought would be to develop something like a horizontal Gowlandflex that could be both your studio and field camera. They could include a focus indicator on the camera to help.

Good luck.

bglick
6-Aug-2005, 16:49
Kevin, I appreciate your input..... I was trying to prevent this thread from becoming a DOF study, as my goal was to build a camera and was trying to get help with the mechanics of such.... I will provide one simple example that demonstrates the critical focus issue, even on scenes with infinity, as I know this a a bit confusing, and till I started toying with the math long ago, I too did not realize just how critcal focus is - assuming you place a high value on the "near / far resolution" recorded. In stereo, this is ultra critical for crisp looking views.

Here is a scene with 47mm lenses, with infinity that meets a set of min. requirements....

47 focal length mm

11 f number

3.0 lpmm film, near/far, after 4x magnification

16 dxf distance to exact focus, feet

So.......

16 Hyperfocal distance feet

8.0 DOF N ft (meets min. lp/mm requirements)

-1349 DOF F ft (meets min. lp/mm requirements)

Agreed? Simple DOF / hyperfocal math.... right? Now, lets say I mis focus the rail by exactly .1 mm . This will change my focus point to 13 ft. (to long to display the mis focus math) So here is the numbers for the same scenario but mis focussed on 13 ft vs. the desired 16 ft.

47 focal length mm

11 f number

3 lpmm film, near/far, after 4x magnification

13 dxf distance to exact focus, feet

So......

7.1 DOF N ft (meets min. rez requirements)

73.1 DOF F ft (meets min. rez requirements)

Obviously this misses my desired "on film resolution" goal....... to reverse engineer how bad........ it will reduce infinity resolution to 2.4 lpmm.... not too bad, but lets take something more realistic, I mis focus on the rail by .25mm, (about the thickness of standard copy paper, 20 lb text) this pulls my focus point to 10 ft.... now....

47 focal length mm

11 f number

3 lpmm film, near/far, after 4x magnification

10 dxf distance to exact focus, feet

So....

6.1 DOF N ft

27.2 DOF F ft

Now infinity will be at 1.8 lp/mm after magnfication.... almost half what my desired goal is....

And I think you would agree, mis focussing the rail the thicknesss of a "piece of paper" would be a best case scenario, not a worst case scenario. With all the other variables in the system, it's easy to be "off" by more than double this, missing my target "on film resolution" in near / fars, up to a factor of 3. This is why precision focus is so critical....its not always the mis focussed subject at the focus distances that will suffer, but rather the near /far resolution that suffers. This is one of the issues I want this camera to overcome vs. other stereo cameras. (+ lens sync)

By using highly leveraged focussing helicals such as suggested by Struan, it can overcome 90% (?) of these precision focus issues. The point is, the human factor still exists, but with the proper mechanical geared focus system in place, the human now has sufficient eye/finger dexterity to make the focus accurate, whereas with standard view camera rail focus, the eye / fingers / gearing, are not precise enough to acheive the accuracy one needs to assure desired "on film resolution", specifically at the near/ far zones. Make sense now why I am going through all this trouble? I have tons of LF gear laying around to use without buying anything! Yet none can accomplish this task.... A MF camera is ideal, as the engineering is complete for this task, but they can't come close to the center lens spacing I need. So, any suggestions other then what we have been exploring?

Paul Fitzgerald
9-Aug-2005, 09:13
Hi there,

Bill, ebay item #7537161230, the answer to your prayers.

Have fun.

pico
9-Aug-2005, 19:28
Yes, a conventional helix focusing mount won't work with a dual-lens (stereo?) camera.

I believe you have a solution: mount the two lenses onto a common board with a block that is prefocused at Infinity. Make the block removable, possibly with wing-nuts. Make, or have made, the proper spacer to go under the block (a block with a hole in it) so that it focuses at 20'. Make other for closer work if you like.

Never mind the nay-saying of the stridents. It is an inexpensive fix and will work perfectly well. If you are using 47mm F8 S/A lenses, stop down to F16 - they are not excellent wide open.

I hope this is helpfull.
P

bojove
18-Jan-2012, 19:55
BJ, I have used helical focusing mounts for Super Angulon lenses in 65mm, 75mm and 90mm. I have had good results with the Chinese made helical that is sold on eBay, and I have also used the helical from the old Mamiya Press cameras. Here is a link that shows the cameras that I have made with those mounts. http://www.flickr.com/photos/bojove/
For the 47mm I just set the hyper-focal at 12" and stopped it down to control focus. Have Fun, B~

Leigh
18-Jan-2012, 20:40
bojove...

This thread is 6 1/2 years old.

- Leigh

jb7
19-Jan-2012, 02:30
It'll work with a Tessar, I'm not sure what other designs might work.
Also don't know what the focus range will be before your front element drops out and smashes...

6 years Leigh- just noticed that, and I only read the first page-
someone's probably said this already...