New to tings and swilts, I "kinda" get it

Discussion in 'Large Format Cameras and Accessories' started by Rich Ullsmith, Jan 14, 2010.

  1. Rich Ullsmith

    Rich Ullsmith Member

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    Apologize up front for the spate to of stupid questions I will be peppering you guys with in the coming weeks/months.

    Finally got my 4X5 Calumet all together, culled a boatload of film and half-dozen good carriers and am now waiting for some sunshine. I have exposed processed 20 or so sheets . . .first dozen were destroyed in a darkroom fiasco that I can't get into right now. If there's a million ways to screw it up, I'm on about #4.

    I do have a couple of books on LF, so I have some clues. Question #1: Is it necessary to refocus after stopping the lens down? I understand the depth of field will change, but will the actual focal point change at all? I really have a tough time seeing anything stopped down to f16 or less. Maybe I need a heavier dark cloth, or maybe this is just the nature of the beast.
     
  2. Christopher Walrath

    Christopher Walrath Subscriber

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    I would say that, except in extreme close-up work, the increased DOF should not prove to be a problem at all. You've already zeroed in on your (presumably) shallowest DOF. Increasing it only stands to improve focus. I would think.
     
  3. keithwms

    keithwms Member

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    Not usually necessary, but sometimes it is. A well-corrected lens should give consistent focus through all apertures, and almost all modern lenses do. But let's suppose the focus shifts a bit, then the effective increase in DOF that you get by stopping down will usually take care of that.

    It is a bigger issue with enlarging lenses, I saw pretty substantial focus shifts when stopping one down. I discarded that particular lens.
     
  4. OP
    Rich Ullsmith

    Rich Ullsmith Member

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    No, I haven't gotten into the bellows factor thingy yet. The guidance I have in one of the books is you can count on from camera to subject, you can count on 1/3 in front of subject and 2/3 behind to be in focus.
     
  5. keithwms

    keithwms Member

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    My bellows factor method is as follows. First of all guess/measure the magnification factor. Let us call that m. For example if the subject is 1:1 on your ground glass, then m=1.

    Multiply your exposure by (1+m)^2. E.g. for a 1:1 shot, your bellows factor is (1+1)^2=2^2=4... meaning that your exposure needs to be 4 times longer.

    Obviously if m is a small number then no additional exposure will be required.

    What I do to get m accurately is put a piece of paper or coin next to my subject and compare it to one held up to my ground glass. If you want to be really accurate you can use a ruler. Ruler markings on your GG can be helpful in this regard.

    There are all kinds of methods, of course. But this has never failed me and requires no gear or measurements, just eyeballing m will almost always get you close enough. For slide film you may wish to measure more carefully. For b&w, nah, it's easy just to guesstimate.

    Things only get complicated when you have substantial movements and need to factor in falloff/Scheimpflug.
     
  6. bdial

    bdial Subscriber

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    That range is dependant on the f/stop you are using.
    As Keith said, for modern lenses the focus shouldn't shift, though, you may want to check to verify that everything looks in focus that you want sharp.
    You may want to adjust your focus point to make the best use of the range you have available at a particular stop. In that case, you'd open up, focus on the new point, then stop down again and see how things look.

    Jason Brunner has an entertaining video on dealing with bellows factor.
     
  7. keithwms

    keithwms Member

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    Wait, might you be confusing bellows factor with hyperfocal distance? I don't see what 1/3, 2/3 has to do with bellows factor. Bellows factor is just additional exposure you have to give because the light from the subject is getting cast over a wider area. Hyperfocal distance is a helpful quantity when you want to determine the near and far focus points... and to avoid having to stop down excessively (which incurs diffraction softening).
     
  8. keithwms

    keithwms Member

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    P.S. Oh, maybe you are thinking of measuring the length of the bellows versus the FL of the lens, that is another way to get bellows factor. Yeah some people do that with string or make markings on their rails.
     
  9. jeffreyg

    jeffreyg Subscriber

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    A suggestion re bellows factor: take a calculator and figure out the exposure increases for the extension for each half stop for each lens. Get a couple of small retractable tape measures and at the appropriate inches (centimeters) mark the factor (.5 ,1.0 ,1.5 ---) with a permanent marker. Then when you are set up to take a photograph, measure from the lensboard to the film plane and you will know what increase is needed.
     
  10. OP
    Rich Ullsmith

    Rich Ullsmith Member

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    Sorry to digress about the bellows factor. What I meant was, until I get a little more proficient I don't think I'll be doing any close-up shots.

    I got a pretty detailed PM about it, and my basic understanding is with older lenses, because of spherical abherration the focal point will shift, but with a long camera-to-subject distances it is not significant.