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Thread: Diffraction. When does it really matter with LF?

  1. #1

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    Diffraction. When does it really matter with LF?

    Hi.
    I'm not 100% sure this is the right spot for this question, so please feel free to move this. It's a technique thing....

    Anyway, when looking at moden digital gear, I believe the generally accepted "fact" is that diffraction starts to creep in at about f/16. For those egale-eyed folks that might be true, but for me, not so much...I can say I see a general softening of the image (again digital here) at about f/22 or so.

    I can't say the same for my one and only LF lens which is a Fuji 150mm f/5.6. to me it's super sharp at f/22 and I'm not even sure if there's a problem at f/32.

    So, what's the deal here?

    Do LF lenses tend to preform well at very small apertures, way better than modern digital gear? Is that just how it is, physics?

    Or is diffraction just as prevelent in LF even at f/22 and I just need better glasses and eagle-eye training?

    Thoughts??

    Thx!!

  2. #2

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    Re: Diffraction. When does it really matter with LF?

    Only you can choose between slight diffraction and lack of depth of field. Me? I'll take diffraction!

  3. #3

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    Re: Diffraction. When does it really matter with LF?

    Quote Originally Posted by AdamD View Post
    Hi.
    I'm not 100% sure this is the right spot for this question, so please feel free to move this. It's a technique thing....

    Anyway, when looking at moden digital gear, I believe the generally accepted "fact" is that diffraction starts to creep in at about f/16. For those egale-eyed folks that might be true, but for me, not so much...I can say I see a general softening of the image (again digital here) at about f/22 or so.

    I can't say the same for my one and only LF lens which is a Fuji 150mm f/5.6. to me it's super sharp at f/22 and I'm not even sure if there's a problem at f/32.

    So, what's the deal here?

    Do LF lenses tend to preform well at very small apertures, way better than modern digital gear? Is that just how it is, physics?

    Or is diffraction just as prevelent in LF even at f/22 and I just need better glasses and eagle-eye training?

    Thoughts??

    Thx!!
    The smaller the format the sooner diffraction creeps in. The vast majority of modern lenses for 45 are diffraction limited at f22.
    Modern digital lenses around 11or 16.

  4. #4

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    Re: Diffraction. When does it really matter with LF?

    The rule of thumb is that resolution measured in line pairs/mm is limited by diffraction to 1500/f number. At f/1, the limit is 1500 lp/mm. At f/22 the limit is 68 lp/mm. And so on.

    Another rule of thumb is that a print in which 8 lp/mm is resolved will look sharp at normal viewing distance. This has implications for how much a negative can be enlarged.

    No enlargement (= contact printing) means that a negative shot at f/187 will appear sharp. Enlarging by 10x means that a negative shot at f/19 or so will look sharp. This is why my 35mm Kodachromes (ISO 25) of flowers shot at 1:1 and f/16 nominal, f/32 effective, look fuzzy when printed 8x10.

    The diffraction limit rule of thumb I gave above is a bit lenient. It gives resolution at 0% contrast, resolution with useful contrast is somewhat lower than the rule of thumb suggests.
    Last edited by Dan Fromm; 2-Dec-2020 at 11:49.

  5. #5
    Resident Heretic Bruce Watson's Avatar
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    Re: Diffraction. When does it really matter with LF?

    Quote Originally Posted by AdamD View Post
    So, what's the deal here?

    ...

    Thoughts?
    Everything in LF is a trade off. If you open up too far you have shallow DOF. If you stop down too far you soften the image through diffraction. Etc. The bottom line as you'll find eventually, is to artfully walk all the trade offs.

    What many people have learned before us is that while diffraction limiting makes the entire image slightly softer, DOF works more like a gradient -- sharpness varies across the image. We see the variation in sharpness fairly easily since our visual systems are excellent with patterns.

    This is at least partly why the pioneers like Weston, Adams, Cunningham, etc. who founded Group f.64 chose that name. They valued over all focus. That is, they were willing to give up a little entire image softness to get more entire image DOF.

    How you personally feel about it is of course easy to test -- all you need do is make a number of identical exposures using different f-stops. Make prints from each, put them up on your "viewing wall" side by side and see what you think. The more enlargement of course, the easier it is to see, which of course points you to another consideration. And there are many more considerations out there.

    Bruce Watson

  6. #6

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    Re: Diffraction. When does it really matter with LF?

    Quote Originally Posted by Bruce Watson View Post
    Everything in LF is a trade off. If you open up too far you have shallow DOF. If you stop down too far you soften the image through diffraction. Etc. The bottom line as you'll find eventually, is to artfully walk all the trade offs.

    What many people have learned before us is that while diffraction limiting makes the entire image slightly softer, DOF works more like a gradient -- sharpness varies across the image. We see the variation in sharpness fairly easily since our visual systems are excellent with patterns.

    This is at least partly why the pioneers like Weston, Adams, Cunningham, etc. who founded Group f.64 chose that name. They valued over all focus. That is, they were willing to give up a little entire image softness to get more entire image DOF.

    How you personally feel about it is of course easy to test -- all you need do is make a number of identical exposures using different f-stops. Make prints from each, put them up on your "viewing wall" side by side and see what you think. The more enlargement of course, the easier it is to see, which of course points you to another consideration. And there are many more considerations out there.
    And focused at different points, 1/3rd into the scene, into the scene, on the nearest point wanted sharp, on the furthest point you want sharp.
    And decide how large you will print the scene since dof also varies with magnification.

  7. #7

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    Re: Diffraction. When does it really matter with LF?

    And all of this assumes a lens with a very, very flat field, something not really suitable for general photography. Dagors, for example, were kown for having a "doughnut-shaped" field if not stopped down sufficiently.

  8. #8
    Mark Sawyer's Avatar
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    Re: Diffraction. When does it really matter with LF?

    Quote Originally Posted by Dan Fromm View Post
    No enlargement (= contact printing) means that a negative shot at f/187 will appear sharp. Enlarging by 10x means that a negative shot at f/19 or so will look sharp.
    For large format, this is key. If you're contact printing, (no enlargement of the negative), forget diffraction. It will barely show up at f/256.

    Quote Originally Posted by Dan Fromm View Post
    This is why my 35mm Kodachromes (ISO 25) of flowers shot at 1:1 and f/16 nominal, f/22 effective, look fuzzy when printed 8x10.
    Nitpicking, but isn't 1:1 and f/16 nominal really f/32 effective?
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  9. #9

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    Re: Diffraction. When does it really matter with LF?

    Quote Originally Posted by Mark Sawyer View Post
    For large format, this is key. If you're contact printing, (no enlargement of the negative), forget diffraction. It will barely show up at f/256.



    Nitpicking, but isn't 1:1 and f/16 nominal really f/32 effective?
    You're right. Stupid typo, corrected. Thanks for spotting it and telling me.

  10. #10
    Jim Jones's Avatar
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    Re: Diffraction. When does it really matter with LF?

    There are many rules for optimum apertures, but my favorite is: stop down until the iris appears to be about 1/4 inch in diameter when viewed through the front lens cell. This applies equally well to all film sizes. Some subjects, some lighting conditions, some personal preferences, and all practical pinhole photography demand exceptions.

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