Psuedo helical focussing - possible?

[bglick]

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]

"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]

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]

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]

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]

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]

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]

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]

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]

Here's a similar project:

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

Good luck!

David