Helical Focusing for P&S Wide Angle 4x5

Discussion in 'Camera Building, Repairs & Modification' started by Terrance Hounsell, May 29, 2006.

  1. Terrance Hounsell

    Terrance Hounsell Member

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    I am in the process of building a Wide Angle 4x5 Point and Shoot. I have the aluminum body, back, tripod socket, accessory shoe, level, lens, shutter and aluminum lens board all in place and need to resolve one last detail before putting the final touches---->Focusing

    The lens I am using is a Nikkor 65mm f/4 in Copal #1 shutter mounted in a recessed lensboard which is reversed. When everything is placed into position I have 40mm between the film plane and the nodal point of the lens thus I have a 25mm gap at infinity.

    How to fill the gap? Well, I could make the camera fixed focus and rely on hyperfocal distance for focus but I am minded to use a bit more finesse than that. Especially since I am guessing (but will check it out on my Wisner tonight) that the actual movement of the lens from say 5 feet to infinity is probably only 5-6mm.

    So I am looking for a helical focusing mechanism (approx 1" thick). There is a chinese one on eBay that is stated to be available for a 65mm in copal #1. I have emailed the seller to inquire about dimensions and availability but the language barrier has left me unsure... I will continue to persue this possibility.

    Meanwhile I would appreciate advice on any other options or sources for suitable helical focusers.

    Movements are not really an issue with this camera, and I'll probably end up shooting 6x12 format on 120 in the long run. Thanks in advance to all.
     
  2. Dan Fromm

    Dan Fromm Member

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    Huh? You went ahead and cut metal before knowing what you were going to use for focusing? Oh, my.

    Rodenstock and Schneider sell focusing helicals. I'm not sure who handles 'em in the US. You might try the usual suspects (Badger Graphics, B&H, MPEX, also SKGrimes). Also, they turn up from time to time on, yes, eBay.
     
  3. pelerin

    pelerin Member

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    Hi,
    Some possibilities are:
    1: Schneider and Rodenstock both offer helical focus mounts
    2: SKGrimes, check the website, offers a helical focus mt and could obviously do any machine work to achieve a proper fit etc.
    3: Buy a used slr lens that has other problems (e.g., fungus) and salvage it for the focus mt. I have tried this with a Nikon lens and found that you could extract the parts you need without to much difficulty.
    4: Black PVC pipe nipple + threaded end cap + your machining = crude focus mt. I have examined a camera built this way and, if you don't object to the lack of finish, the main problem is that the lens rotates when you focus.
    Celac.

     
  4. BrianShaw

    BrianShaw Member

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    Not trying to discourage you from putting a focusing mechanism on you camera, but isn't the definition of P&S somewhat inclusive of fixed focus but not focusing helicals?
     
  5. Terrance Hounsell

    Terrance Hounsell Member

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    Helical Focusing Dilema

     
  6. BrianShaw

    BrianShaw Member

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  7. Dan Fromm

    Dan Fromm Member

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    Hmm. How to prevent droop?
     
  8. sanderx1

    sanderx1 Member

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  9. jacobus

    jacobus Member

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    You may also look at a Pentax "Helicoid Extension Tube". There are 2 models, a smaller one having M42 on both sides with an extension range of 17 to 35cm, and a bigger one for the Pentax 6x7 .
     
  10. Terrance Hounsell

    Terrance Hounsell Member

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    Low Profile Helical is important

    The Nikkor 65mm f/4 is not a large lens but the distance from the centre of the shutter to the rear element chews up alot of of the 65mm required for infinity focus. Some helicals can only be mounted entirely behind the lens using a rear element to helical adapter. However, a short focal length the lens cannot be focused as it will be too far away from the film plane. Another problem is that in wide angle designs the rear mounted focuser will cause mechanical vignetting.

    Thus what ever the solution is is will have to be low profile. I only have about an inch to play with and this means that I'll need something that fits between the lensboard and the shutter. The rear elements will need to pass through the helical and lensboard and thus use slightly less than 1/2 of the space available for focusing.

    In reply to the previous question about how to control droop of the gasket material for a two position focusing solution: this is not a problem with a medium density foam (1.5" x 1") cut from a 5x5" sheet 1.5" thick. Dense enough to be light tight but still compressible to aloow a change of focus.

    Two position focus example:
    1) from 14" to 20 feet.
    2) from 20 feet to infinity.
    This way you are not compromising on the circle of confusing as much and the focus will be sharper (and shutter speeds higher as a result of larger aperture).

    The other advantage of a thick foam gasket would be that it could be compressed more on the bottom than top to give a bit of lens tilt to aid in depth of field.
     
  11. fotoman

    fotoman Member

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    Fotoman Camera sells a complete range of Helical Focus Mounts, available to accept either #0 or #1 shutter sizes. A 65mm is available with proper DOF scale. www.fotomancamera.com
     

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  12. noseoil

    noseoil Member

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    Focusing for idiots (thought morons was a bit much), me included

    I had posted this image to another thread about home made cameras, but then deleted it. I'm posting it again as the band width is now clear. The focusing is done simply by moving an inner box inside an outer box. Parts were taken from a Polaroid 110 camera. The gasket mounted in the inner surface of the front of the outer box is "fuzzy velcro," while the gasket on the outer side of the back of the inner box is "fuzzy velcro." The lens board is mounted on the inner box after the lens is mounted on it, and keeps the box from sliding into the camera. The velcro keeps the boxes from sliding past each other as the lens is focused at closer distances. tim

    P.S. This is the best explanation I can give. Draw it on paper and you will see how easy it is to make and use.

    P.P.S. Uploaded image to technical gallery, not sure why it wouldn't load into the thread.
     
    Last edited by a moderator: Jun 2, 2006
  13. jacobus

    jacobus Member

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    "whatever the solution is it will have to be low profile ..."

    This constraint would not exist if you could find a helical focuser whose inner diameter is wide enough to accept the rear cell of your lens. Since I do not know the exact specs of your lens I cannnot be certain, but I think a Pentax 6x7 "Helicoid Extension Tube" with a max.outer diameter of 89mm should be adequate to accept a rear cell of only 55mm dia.
    And with a minimal extension of 32mm ( max. 52mm) you wouldn't risk vignetting.
    There is some information about this "tube":
    http://www.pentax.com/docstore/index.cfm?show=6

    The good thing with helical mounts ( apart from not being limited to pre-set distances): No trouble with collimation.