Circle of confusion in LF

Discussion in 'Large Format Cameras and Accessories' started by afrank, Sep 11, 2012.

  1. afrank

    afrank Member

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    Hi all,

    Since I am new to LF I think I really have to deal with the Circle of Confusion that is driving me in circles to the point of confusion :D.

    So for a specific lens in 4x5/9x12, lets say 130mm the CoC is about 0.1 mm no matter what f-stop or distance to subject. (in this calculator atleast: http://www.dofmaster.com/dofjs.html)
    I thought there was a f-stop sweet-spot that you had to step down to, in order to get a decent infocus/detail/sharp image...

    For such a camera 4x5 with 130mm what would that be? My film plane seems to be very reliable since my film plates have the springs and holders that keep the film flat and straight.

    I know about DOF and F-stop, hyper-focal distance, etc, but this sharpness on the image plane is killing me!

    Ty!
     
  2. Prof_Pixel

    Prof_Pixel Member

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    The CofC in that calculator is simply used for calculating the near and far focus points (depth of field)

    Notice that as you change the f stop or focal length, the near and far focus points change

    What you may be thinking of is the Circle of Least Confusion which is the smallest such spot that a lens can produce
     
  3. Bill Burk

    Bill Burk Subscriber

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    You want to have points at the various distances... be focused as small as 0.1mm.

    Basically you are saying that's as fuzzy as you can stand it... and still call it in focus.

    I don't want to make you miserable, but really soon you start to reach the "diffraction limit."

    After you hit the diffraction limit, you get general fuzziness that you might recognize from seeing pictures taken with a pinhole camera.
     
  4. markbarendt

    markbarendt Subscriber

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  5. wiltw

    wiltw Subscriber

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    The Circle of Confusion is the size of a blurry disk (blurred representation of a precise point which is not perfectly in focus) which a person would perceive as a blurry circle if it was any larger than that. In fact, CofC dimensions are 'manufacturer standard' size, and a person with 20/20 visual acuity would actually be able to perceive that size of blur circle as a blurry circle (the eye and brain of a person with 20/20 vision would NOT be fooled into thinking 'sharp point'!)

    The size of the CofC has nothing to do with FL...if your eye detects a blurry circle, it would not matter if that blur were captured with a 13mm lens or 130mm lens or 1300mm lens.
     
  6. ic-racer

    ic-racer Member

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    The CoC is YOUR parameter based on how close you view YOUR prints. It has nothing to do with the lens and it can be whatever you want.
     
  7. markbarendt

    markbarendt Subscriber

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    I agree.

    The best way I can think of to describe it is as a tolerance, as in an industrial process or for a machine part.

    The tolerance, COC, is a variable that we can define dependent upon our needs.
     
  8. Alan Klein

    Alan Klein Member

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    I shoot MF not LF. Regardless, what I do when I'm setting the aperture, I calculate the DOF I want things sharp. Then, I stop down one more stop for good measure.
     
  9. Prof_Pixel

    Prof_Pixel Member

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    From the Kodak Professional Photoguide (Fall, 1975): Technically, the circle-of-confusion size used to calculate the depth of field for each situation is about 1/1000 of the focal length of the normal lens for each format.
     
  10. afrank

    afrank Member

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    Thanks all for the info! I was clearly using CoC wrong! It helped clear things up!
     
  11. summicron1

    summicron1 Subscriber

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    the depth of field calculator on my combat speed graphic with a 127mm optar (ektar) uses a 1/300th of an inch circle of confusion assumption. With that it says, for example, that you are in focus from 21feet 6 inches feet to 29feet 7inches when the lens is set at f 4.5. It doesn't say how large a print that assumes, however, but I would guess an 8 by 10 inch. The larger the print, the less depth of field is safe to assume, as others have said.

    I'm something of a pragmatist about these things -- or maybe subjectivist is a better word. Play around until you find a method that gives you nice images and don't worry about the technical aspects of things.
     
  12. markbarendt

    markbarendt Subscriber

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    The problem I see with the fixed number model examples above, 1/300 or 1/1000, is that only the lens and film are considered.

    Its a guess, but I think a reasonable one, that those numbers were generated based on "normal" print size, viewing distance, and expectations.