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  1. #1

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    CoC and depth of field

    A question about balancing depth of field and resolution. Bear with me this might be confusing if I don't write it out clearly enough. Assume 35mm negatives to be enlarged to around 8x10".

    In my photographs I want everything sharp (read no selective depth of field). At the same time, whenever possible I prefer not to use apertures smaller than f11 as f16 seems to be where diffraction effects become readily noticeable. So there is always a balance that must be struck. I've started using my brother's nifty golf rangefinder to measure the distances in a scene before focusing. So I'll measure distances to the far away objects, the near objects, and/or the most important objects.

    Now, most depth of field tables, including the scales on most lenses (which are not necessarily even accurate) assume a CoC of 0.03 or 0.032 as far as I can tell. But for film that needs to be enlarged a CoC this large is really pushing the limits of what I would consider acceptable sharpness. So I usually try to use a smaller CoC, arbitrarily something like 0.02 or lower if possible (I use the DoFMaster website to calculate depth of field for different CoC sizes using my distance measurements).

    What I'm never sure about is whether to favour a smaller CoC with less depth of field or a larger CoC with more depth of field. Which will result in the higher perceived sharpness (all other factors remaining equal)?

    Here's an example. Suppose for a scene I've determined f11 gives sufficient depth of field for a CoC of 0.025. Would it be better or not, to stop down to f16 (ie more diffraction) and theoretically get a CoC of 0.015 (ie more depth of field)?

    And how much smaller does a CoC have to get from the standard 0.03 for there to be a visible improvement in resolution?

    Is 0.02 significantly better than 0.03?

    Is 0.01 significantly better than 0.03?

    What is the lower cutoff point beyond which we can't really see an improvement (assuming a constant optimum aperture)?

    Obviously the enlargement factor plays a major role...

    Michael

  2. #2
    Bill Burk's Avatar
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    I don't know if this is oversimplifying things, but I just set the aperture at f/16 and use the scales for f/11.

  3. #3
    David A. Goldfarb's Avatar
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    There are various ways to calculate the value for acceptable CoC, but the standard values vary by format. Bigger format, smaller enlargement factor for the same print size, so a larger CoC is going to be visually acceptable.

    Now, if you're like me, you might want things a hair sharper than the standard values, and rather than enter into a complex calculation, I do exactly what Bill Burk recommends above, with the same result: I stop down one or two stops from what the scale or table says to use for the DoF I want, and I don't really worry about diffraction, unless I'm shooting macro at fairly high magnification, where the effective aperture is going to be significantly smaller than the value engraved on the lens. Inadequate DoF when you need it is a bigger visual distraction than diffraction at non-macro subject distances.
    flickr--http://www.flickr.com/photos/davidagoldfarb/
    Photography (not as up to date as the flickr site)--http://www.davidagoldfarb.com/photo
    Academic (Slavic and Comparative Literature)--http://www.davidagoldfarb.com

  4. #4

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    Quote Originally Posted by David A. Goldfarb View Post
    Inadequate DoF when you need it is a bigger visual distraction than diffraction at non-macro subject distances.
    I guess this is really what I'm getting at. Is that statement always true? There is likely a crossover point beyond which diffraction obliterrates whatever additional detail you gain by increasing depth of field. On the other hand, if any amount of blur resulting from defocus always looks worse than any amount of fuzziness from diffraction, then I guess the statement would indeed always be true - meaning for sharper prints you are always better off decreasing the size of the CoC than maximizing resolution. Is this true?

  5. #5
    ic-racer's Avatar
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    Quote Originally Posted by Michael R 1974 View Post
    A question about balancing depth of field and resolution. Bear with me this might be confusing if I don't write it out clearly enough. Assume 35mm negatives to be enlarged to around 8x10".

    In my photographs I want everything sharp (read no selective depth of field). At the same time, whenever possible I prefer not to use apertures smaller than f11 as f16 seems to be where diffraction effects become readily noticeable. So there is always a balance that must be struck. I've started using my brother's nifty golf rangefinder to measure the distances in a scene before focusing. So I'll measure distances to the far away objects, the near objects, and/or the most important objects.

    Now, most depth of field tables, including the scales on most lenses (which are not necessarily even accurate) assume a CoC of 0.03 or 0.032 as far as I can tell. But for film that needs to be enlarged a CoC this large is really pushing the limits of what I would consider acceptable sharpness. So I usually try to use a smaller CoC, arbitrarily something like 0.02 or lower if possible (I use the DoFMaster website to calculate depth of field for different CoC sizes using my distance measurements).

    What I'm never sure about is whether to favour a smaller CoC with less depth of field or a larger CoC with more depth of field. Which will result in the higher perceived sharpness (all other factors remaining equal)?

    Here's an example. Suppose for a scene I've determined f11 gives sufficient depth of field for a CoC of 0.025. Would it be better or not, to stop down to f16 (ie more diffraction) and theoretically get a CoC of 0.015 (ie more depth of field)?

    And how much smaller does a CoC have to get from the standard 0.03 for there to be a visible improvement in resolution?

    Is 0.02 significantly better than 0.03?

    Is 0.01 significantly better than 0.03?

    What is the lower cutoff point beyond which we can't really see an improvement (assuming a constant optimum aperture)?

    Obviously the enlargement factor plays a major role...

    Michael
    Well informed question. You have to take pictures at the various apertures with your system (or look back at at your records) and calculate backwards your acceptable CoC. Again, you have asked a good question and not many people seem to understand that 'acceptable CoC' is a personal value.

  6. #6

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    Agreed. I was actually just thinking about trying a few real world tests. It could make for an informative discussion.

    Michael

  7. #7

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    Depth of field depends on seven factors, not just the four that most people think of. In addition to focal length, f/stop, and distance, DoF depends on film size, print size, viewing distance, and visual acuity.

    This is the only calculator that I know of that applies all seven: http://eosdoc.com/jlcalc/ Explore it and vary all 7 variables.

    You can't just go with a given size for the CoC.

    Quote Originally Posted by Michael R 1974 View Post
    Assume 35mm negatives to be enlarged to around 8x10".
    Then go with the results of the DoF calculator and/or the DoF markings on the camera lens if it has any.
    Last edited by Monito; 09-09-2011 at 06:32 PM. Click to view previous post history.

  8. #8
    MattKing's Avatar
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    This thread would make a great "Sticky".
    Matt

    “Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”

    Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2

  9. #9
    David A. Goldfarb's Avatar
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    Quote Originally Posted by MattKing View Post
    This thread would make a great "Sticky".
    Good idea. I just stuck it on my iPhone using Tapatalk. Did it work?
    flickr--http://www.flickr.com/photos/davidagoldfarb/
    Photography (not as up to date as the flickr site)--http://www.davidagoldfarb.com/photo
    Academic (Slavic and Comparative Literature)--http://www.davidagoldfarb.com

  10. #10
    MattKing's Avatar
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    Quote Originally Posted by David A. Goldfarb View Post
    Good idea. I just stuck it on my iPhone using Tapatalk. Did it work?
    It sure did.
    Matt

    “Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”

    Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2

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