Psuedo helical focussing - possible?

[Paul Fitzgerald]

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]

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]

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]

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]

Hi there,

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

Have fun.

[pico]

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]

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]

bojove...

This thread is 6 1/2 years old.

- Leigh

[jb7]

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...