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Thread: Psuedo helical focussing - possible?

  1. #1

    Join Date
    Feb 2002
    Posts
    781

    Psuedo helical focussing - possible?

    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

  2. #2
    Doug Dolde
    Guest

    Psuedo helical focussing - possible?

    A helical focusing mount moves the whole lens. Let me have some of what you're smoking dude.

  3. #3

    Psuedo helical focussing - possible?

    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

  4. #4

    Join Date
    Feb 2002
    Posts
    781

    Psuedo helical focussing - possible?

    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.

  5. #5

    Psuedo helical focussing - possible?

    focus = movement. The only option is fixed hyperfocal and heaps of f-stop

  6. #6

    Psuedo helical focussing - possible?

    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

  7. #7

    Psuedo helical focussing - possible?

    With 35mm and MF lenses, various elements in the lens move in relation to each other when you rotate the focussing ring.

  8. #8

    Psuedo helical focussing - possible?

    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.

  9. #9

    Join Date
    Feb 2002
    Posts
    781

    Psuedo helical focussing - possible?

    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

  10. #10

    Join Date
    Jun 2005
    Posts
    2,480

    Psuedo helical focussing - possible?

    "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.

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