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  1. #21
    RalphLambrecht's Avatar
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    the nice thing about working with cocs s that the enlargement factor does not play a role any longer.
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

  2. #22
    RalphLambrecht's Avatar
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    viewing distance and print size cancel each other out and are no longer varibles if a coc is uded.
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

  3. #23
    andrew.roos's Avatar
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    In his book Post Exposure Ctein suggests that "adequately sharp" (meaning, it appear sharp when viewed alone) requires about 5 lp/mm on an 8x10 print, but that "perfectly sharp" (meaning no difference is visible in side-by-side comparisons if sharpness is increased further) requires about 30 lp/mm! On an uncropped 35mm negative (enlargement factor 7) this corresponds to CoC diameters of about 0.014 mm and 0.002 mm respectively, but the latter is not achievable due to film, lens sharpness and DOF limitations. For "adequately sharp", an aperture of f/16 should give an airy (diffraction) disk diameter of around 0.014mm.

    My own inclination is to retain diffraction sharpness and compromise DOF if necessary since I can adjust the focus distance to ensure that the aspects of the scene that I consider most important - typically those closest to the camera - are as sharp as possible, while allowing some loss of sharpness for (due to DOF limitations) in other areas. If you start to lose sharpness due to diffraction, then this affects everything in the photograph, which I find the worse of the two evils.
    Last edited by andrew.roos; 04-04-2012 at 03:16 PM. Click to view previous post history.

  4. #24

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    An interesting read on the subject can be found here....
    http://www.normankoren.com/Tutorials/MTF6.html

    Enjoy!
    Anyone can make a Digital print, but only a photographer can make a photograph.

  5. #25
    RalphLambrecht's Avatar
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    Michael,I have a must-read for you on this topicbut it's too big for APUG.please send me a n email to rlambrec@ymail.com.
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

  6. #26

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    [QUOTE=Monito;1234252]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.

    Any chance of an update on that eosdoc website - it reads page not found now...

  7. #27
    PeterB's Avatar
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    [QUOTE=M6F6E6;1953722606]
    Quote Originally Posted by Monito View Post
    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.

    Any chance of an update on that eosdoc website - it reads page not found now...
    Here is the last known good version captured by the Wayback Machine, the amazing www archiving site.

    http://web.archive.org/web/201209151...doc.com/jlcalc

    I even saved a copy of that page locally and it still worked as the javascript is also saved.

  8. #28
    RalphLambrecht'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
    forget Coc and DOF and concentrate on shooting with your lens' sweet spot instead.making photographs that are sharp from front to rear without selective sharply focused areas is optically impossible because real focus is only available within the focus planeDOF and COC' cheat' with acceptable sharpness ,assuming a proportional viewing distance,which is OK if you can keep the observer from sticking their noses onto your print
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

  9. #29
    RalphLambrecht's Avatar
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    Quote Originally Posted by Bill Burk View Post
    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.
    this helps with DOF but pushes you deep into diffraction terretory.used with landscapes, it will always push infinity to the threshold of sharpness.wider apertures give more resolution and detail than small apertures.stopping down to gain sharpness is a very commonfallacy.
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

  10. #30
    RalphLambrecht's Avatar
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    Quote Originally Posted by David A. Goldfarb View Post
    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.
    interesting;my experience is very different.after sticking to f/8-11 for 35mm, my photographs are visibly sharper.IMO the influencediffraction is underated
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

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