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  1. #11
    clay's Avatar
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    I agree that f/22 optimizes the sharpness across the field, but it does so at the cost of absolute sharpness in the center. Check out this mtf graph of a tele-xenar (tessar design) from Schneider:

    http://www.schneideroptics.com/pdfs/...56_250%202.pdf

    and look at the last two graphs showing mtf from f/11 and f/22. The f/22 is more uniformly sharp across the field, but absolutely less sharp in the center than at f/11. The difference is so small that it is of no practical consequence, but the fact is that if you want the sharpest possible image in a subject in the middle of your frame, you would be better off shooting at f/11 than at f/22, even with a tessar. But again, in practical terms, it is of no consequence, unless you are shooting with tech pan on your 4x5.

    Quote Originally Posted by Ian Grant View Post
    Clay, the Tessar's were designed to give maximum sharpness & resolution at f22, somewhere I have the figures from Doctor Optic who were the last manufacturer of the CZ Jena designed Tessar's, resolution tailed of significantly after f32 due to diffraction for the135mm/150mm lenses.

    Ian
    I just want to feel nostalgic like I used to.


    http://www.clayharmon.net - turnip extraordinaire

  2. #12

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    Well it's not entirely about DoF. The idea is to find the aperture that will give you the sharpest image for the depth of field required. It's a trade-off between a small enough aperture to create the DoF you want and the diffraction caused by stopping down too far. Since I saw this my taking technique has been to find the bellows extension difference between near and far and then consult the chart for the "best" aperture. My shutter speed falls out of all that. Kinda like Large Format Aperture Priority!!!!
    "Why is there always a better way?"

  3. #13
    keithwms's Avatar
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    Bob, for LF my strategy is this. Starting with a wide aperture I focus and then attempt to gain sufficient coverage and sufficient effective depth of field via tilts and shifts. Failing that I stop down. Then... rinse, repeat... no formulas, no math In LF you are almost always working on issues other than resolution.

    f/22 is certainly already incurring diffraction softening. Most LF lenses are giving maximum resolution values across the frame at around f/16 or even f/11. Indeed f/22 is the aperture at which the lens typically reaches highest coverage, not the highest resolution across the image circle.

    In MF, lacking the tilts, you wind up stopping down more than would be necessary if you did have tilts. C'est la vie.
    "Only dead fish follow the stream"

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  4. #14

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    This article on the LF photography site covers what you're looking for, and there are links to the Photo Techniques article under "References" at the bottom of the page:

    http://www.largeformatphotography.info/fstop.html

    Jim

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