Where to set a lens?

Discussion in 'Camera Building, Repairs & Modification' started by EdColorado, Jun 5, 2014.

  1. EdColorado

    EdColorado Member

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    I'm thinking of building, and am working on the design for, a fixed focus 6x17. I'm trying to decide what distance to set the lens from the focal plane. My first thought was of course to just set it at the lenses proper flange focal distance but now I'm wondering if that is the best idea. Can I set it to a closer focus point to try and take advantage of the hyper-focal distance to gain a wider depth of field? How do I determine just what that new focal distance would be? The lens I'll be using is a 180mm Rodenstock Sironar-N. If what I read on the web is correct, it has a flange focal distance of 174mm.

    Thanks much for any input,

    Ed
     
  2. jon koss

    jon koss Subscriber

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    Hmmm… my gut reaction is that a 180mm lens is quite long for this type of project. Something in the 120mm +/- suggests itself at first blush.

    Jon
     
  3. EdColorado

    EdColorado Member

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    You could be right Jon but a 180 is what I have. It is considered a "normal" lens for the 6x17 so I figure to give it a try. My design will allow for changing to a different lens and lens holder and after using the 180 I may well be looking for something a bit shorter. I'll be making the camera body shallow enough to allow me to mount something down to a 90mm. Longer lenses just need a longer mounting box to allow for their longer flange focal distance.
    I'm sorta thinking about how I might incorporate some kind of dark slide to allow for changing lenses mid roll but I'm thinking its starting to get out of hand here and a roll is what, four frames? Probably not worth the effort and complication.
     
  4. NedL

    NedL Subscriber

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    I'm no expert and someone else might have a better idea. I thought about this when I made my fixed focus camera and finally decided against it because I knew that I'd use it stopped down for normal paper negatives and wide open for calotypes. So I opted to set it to infinity. I'm happy with it as it is.
     
  5. pdeeh

    pdeeh Member

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    I would also be very interested to know how to work this out.
    I've built a couple of LF box cameras recently and wanted to try the same, but couldn't get my head around how I would actually do it, so decided on the same approach as Ned ... simple and effective but curiously unsatisfying not being able to understand how to do it the way I wanted :smile:
     
  6. Ian Grant

    Ian Grant Subscriber

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    Personally I feel 180mm is to long for 617, I use a 75mm with mine and to me that's a normal lens. I usually also have a TLR with me with an 80mm lens so in most ways the lenses have similar perspective the essential difference is that in the horizontal plane I have much wider capture, but it's essentially very similar in the vertical plane.

    However I'm not shooting in wide open landscapes like you have in the US so a longer lens may suit your needs, I actually found a 90mm a louch long in the landscapes I work in in Greece & Turkey.

    You can get a Chinese focusing helical quite cheaply or do a search and see how Steve Smith made one from an old lens, that would be a better option than relying on hyper-focal distance with a 180mm lens. Most point and shoot cameras use wide angle lenses.

    Ian
     
  7. Xmas

    Xmas Member

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    Given you only have a few shots per roll you could set the lens at infinity and 3d print extension tubes for a number of distances you would need to provide a sandwich mechanism.

    With three different spacers you get a lot of options ok 8.
     
  8. Jim Jones

    Jim Jones Subscriber

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    I wouldn't trust published flange distances for large format lenses. It's better to check the focus of your lens and camera combination on a ground glass. As for setting the lens to the hyperfocal distance, that distance depends on many factors, such as the aperture used and the acceptable circle of confusion. The latter depends on the most critical use of the image. A large print demands a smaller circle of confusion than an internet image. Photography can be simple, but not if one wants to compete with those who work hard and work smart at it.
     
  9. paul ron

    paul ron Member

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    Since you are building... Why not incorporate some sort of adjustment system? That will allow you to take advantage of dof n enable the use of other lenses.
     
  10. andrew.roos

    andrew.roos Subscriber

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    The thin lens equation might give you a starting point. I don't know whether the corrections required for a "thick" multi-element lens would be significant in this application. The equation is:

    1/O + 1/I = 1/F
    so 1/I = 1/F - 1/O

    where O is the object distance; I is the image distance and F the focal length (all positive).

    According to the Android app DOF Calculator, the hyperfocal distance for a 180mm lens at f/16 is about 17m assuming a CoC of 0.12mm. For an object distance of 1700mm, and focal length 180mm, the image distance (according to the thin lens equation) is 201mm. This means the flange distance must be 201-180 = 21mm further than for infinity focus, so 195mm if the flange distance for infinity focus is 174mm.

    However I would probably focus closer than this, on the basis that some softening of distant objects is an acceptable trade for a closer near focus distance. Of course this depends on the application.
     
  11. cliveh

    cliveh Subscriber

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    What sort of subjects are you intending to photograph with this fixed lens?
     
  12. EdSawyer

    EdSawyer Member

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    First I would get a helical and use that. If that's not practical, set for hyperfocal for your intended aperture (f/16 or smaller presumably).
     
  13. EdColorado

    EdColorado Member

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    Landscapes. Pretty much exclusively landscapes.
     
  14. EdColorado

    EdColorado Member

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    I was thinking of a helical but considering that I'll be using this for landscapes and always on a tripod I don't really think its going to be necessary. At f16 hyperfocal my close focus point (start of depth of field) is somewhere around 8 meters. Stopped down farther and that will come in a bit more. I think that will probably work for the cameras intended use. If not, I can always remake the lens mount and move the lens a bit.
     
  15. pdeeh

    pdeeh Member

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  16. RalphLambrecht

    RalphLambrecht Subscriber

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    determine the depth of focus (not field)for your working aperture and add that to the flange-to-focal-plane distance for max DOF if i'm not mistaken, which happens often.