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Thread: Circle of Confusion and "The Print"

  1. #1
    xpatUSA's Avatar
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    Circle of Confusion and "The Print"

    Having acquired a micro four-thirds camera and a rather fancy macro lens for close-up, it was necessary to re-visit the subject of Depth of Field (DOF) - a subject known and loved by by all, a hardy perennial in these and other fora

    Now, every calculation of depth field requires, as input, the dreaded Circle of Confusion (CoC) or your sensor details or your 'crop factor' so that it can pick one for you. In the latter case, it is of course assumed that you are actually going to print 10"x8" images and inspect them with perfectly average eyes at exactly 200mm distance.

    Enter your non-conformist, complete with myopia. Gazes at images on his or her 86 ppi screen at, say, half a meter. Should he or she look up the CoC in some list somewhere and believe it? tsk!

    I looked up my camera somewhere: it said CoC = 0.016mm; elsewhere it said 0.015mm, close enough, eh? However, for screen-gazing, even with good vision, I calculate it to be 0.037mm !!

    In context, if I shoot a 100mm subject which appears on my sensor as 10mm, a CoC of 0.016 gives me 28mm DOF. On the hand, a CoC of 0.037 gives me 65mm DOF, which could mess up my consummate artistry.

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    Stagecoach's Avatar
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    Re: Circle of Confusion and "The Print"

    Hi Ted,

    Being one to steer well clear of mathematics, formulae and the theory of optics if you have undertaken practical tests to prove your theory can you not simply adjust your aperture to give the DOF you require for your photographic masterpiece

    Grahame

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    xpatUSA's Avatar
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    Re: Circle of Confusion and "The Print"

    That I could, Grahame,

    I was really tilting at the assumption in the literature that we've all got 135 format cameras, shoot for exact amounts of DOF as given by handy-dandy calculators, and live only to view "the print" at the exact right distance under, of course, the exact right lighting while comparing said print with it's file on a perfectly calibrated monitor.

    Unlike in "real life" when such folk are, dare I say, an increasing minority

    Er, I haven't undertaken any practical tests per se, so I really have no idea what aperture is appropriate for a needed DOF. Probably should, eh? I was getting a feel for it with the Sigma SD9 & 10, so it's a bit like starting over.

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    tao2's Avatar
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    Robert (ah prefer Boab) Smith

    Re: Circle of Confusion and "The Print"

    Quote Originally Posted by xpatUSA View Post
    Having acquired a micro four-thirds camera and a rather fancy macro lens for close-up, it was necessary to re-visit the subject of Depth of Field (DOF) - a subject known and loved by by all, a hardy perennial in these and other fora

    Now, every calculation of depth field requires, as input, the dreaded Circle of Confusion (CoC) or your sensor details or your 'crop factor' so that it can pick one for you. In the latter case, it is of course assumed that you are actually going to print 10"x8" images and inspect them with perfectly average eyes at exactly 200mm distance.

    Enter your non-conformist, complete with myopia. Gazes at images on his or her 86 ppi screen at, say, half a meter. Should he or she look up the CoC in some list somewhere and believe it? tsk!

    I looked up my camera somewhere: it said CoC = 0.016mm; elsewhere it said 0.015mm, close enough, eh? However, for screen-gazing, even with good vision, I calculate it to be 0.037mm !!

    In context, if I shoot a 100mm subject which appears on my sensor as 10mm, a CoC of 0.016 gives me 28mm DOF. On the hand, a CoC of 0.037 gives me 65mm DOF, which could mess up my consummate artistry.
    Hi Ted,
    So,
    Ah'm circling in confusion here...you gonna be looking through the camera at the monitor screen? How did ye calculate?

  5. #5
    xpatUSA's Avatar
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    Re: Circle of Confusion and "The Print"

    Quote Originally Posted by tao2 View Post
    Hi Ted,
    So,
    Ah'm circling in confusion here...you gonna be looking through the camera at the monitor screen?
    Yes, that's why they call it "through the lens" lol

    How did ye calculate?
    Hi Baab,

    In the same way as here but using screen-viewing parameters instead of "the print" parameters.

    So, average human vision 0.0011 radians (1.1 mrad) acuity. Half a meter times 1.1 mrad = 0.55mm.

    Now divide by the enlargement which I took to be 254mm wide on my screen enlarged from my 17.3mm wide sensor = 14.68. Therefore CoC = 0.55mm/14.94 = 0.037mm.

    Well, you did ask!

    If I take off my glasses, the CoC increases considerably . . .

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