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Thread: Circle of Confusion query

  1. #1

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    Circle of Confusion query

    I'm looking at hyperfocal distances for a wide angle lens on 10x8 at a fairly small aperture. A 'normal' CoC for 10x8 is (I think) 0.2, whereas a CoC for 5x7 is 0.15.

    Would I get a better result (in real life rather than just theoretically) using the smaller CoC of 0.15 (or even smaller)? Would it even be noticeable in contact prints?

    By my calculations the difference between a 0.2 and 0.15 CoC would move the hyperfocal distance only about 3ft further away from the film plane.

    (I think I understand a little of what the Circle of Confusion is about - but seldom has a technical term been so aptly named )

  2. #2
    Jac@stafford.net's Avatar
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    Re: Circle of Confusion query

    A rule of thumb suggests that you close the aperture (larger numeric ƒ) to gain more DOF. Calculating via CoC just confuses practical expectations. CoC relates more to degree of enlargement which is nil in contact printing. Some people become concerned with diffraction with small numeric apertures, but for contact prints it is not likely critical for any aperture on the lens scale.

  3. #3
    Jim Jones's Avatar
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    Re: Circle of Confusion query

    The CoC is a personal choice. It should be chosen after considering how the final image will be presented, not on what others might recommend. I suspect many photographers would never notice the difference between CoCs of 0.2 and 0.15, but a few certainly will. Remember, a WA pinhole 8x10 camera may have a pinhole of 0.5mm and a somewhat slightly smaller effective CoC, and still produce photos that many consider pleasing.

  4. #4
    Drew Wiley
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    Re: Circle of Confusion query

    I ignore the whole topic. The only thing that counts is how you want the print to look. Unsharpness as well as sharpness is a tool of composition. That's what so wonderful about a big groundglass. And if you need more critical information, a basic loupe will tell you far more than all the hyperfocal formulas in the world.

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    Re: Circle of Confusion query

    Thanks for the reply. Yes, I understand the relationship between aperture and DOF. The application is a box camera so it will have a fixed focal length (I have done this with 5x7 and am now going to build a 10x8, using f32 for the calculations). Looking at the hyperfocal distance gives me a range of 'usefullness' (so to speak). As the CoC is part of the formula for calculating the hyperfocal distance I was wondering what practical effect (if any) reducing the CoC would have on the final image.

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    Re: Circle of Confusion query

    I was replying to Jac@stafford.net and in the meantime two more replies... Maybe my reply makes more sense? Sorry if I wasn't clear in the original query...

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    Re: Circle of Confusion query

    Hmm. On reflection my reply makes no sense - reducing the CoC in the calculation would have absolutely no effect on the final image.

    What it does is moves the hyperfocal distance (slightly) further away (which should be a sharper point) and reducing the band of 'acceptable' sharpness. So is the CoC acting like a kind of resolution tolerance?

  8. #8
    Jac@stafford.net's Avatar
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    Re: Circle of Confusion query

    Quote Originally Posted by peter brooks View Post
    So is the CoC acting like a kind of resolution tolerance?
    Kinda. For 10x8" contact prints use a CoC of 0.2 mm which approximates the resolving power of the human eye under normal viewing conditions. Simplifies things, no?

  9. #9

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    Re: Circle of Confusion query

    Quote Originally Posted by Jac@stafford.net View Post
    Kinda. For 10x8" contact prints use a CoC of 0.2 mm which approximates the resolving power of the human eye under normal viewing conditions. Simplifies things, no?
    Yeah, that makes sense, thanks. I'll stop considering the CoC now

    (side note: the box camera is me convincing myself that I need some 10x8 holders etc... before long I'll convince myself that I need a 'proper' 10x8 camera... You know how it is... )

  10. #10
    ic-racer's Avatar
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    Re: Circle of Confusion query

    Only you know. Just focus 3 feet out and see how the prints appear.

    Also, with the same scene you can go from f90 to f22 and see if you can tell any difference from Airy disk size. You may find (if you do contact prints) f64 works quite well.

    I make 2 power enlargements, (16x20in) and, since I'm nearsighted, I peer at the prints with my nose practically against the paper, so I favor f32 or f22 on my 8x10 camera.

    In terms of "hyperfocal distance" I only focus there by coincidence when the main subject is there and I want the background blurred. For a landscape, I usually focus at infinity as outlined in Merklinger's "Ins and Outs of Focus."

    Page 69, Chapter 10, Rules of Thumb (author's emphasis):
    4. If we want anything at infinity to be critically sharp, focus at infinity.

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