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  1. #11

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    Quote Originally Posted by E. von Hoegh View Post
    Sorry. Diffraction depends solely upon the stop, it has nothing to do with focal length. The diffraction limit in LP/MM can be approximated by dividing the stop # into 1500. At f:45 the diffraction limit is about 30 LP/MM regardless of the focal length of the lens.
    The folks here have a nice write-up on this:
    http://www.cambridgeincolour.com/tut...hotography.htm

    Pity they aren't clear between "aperture" and "aperture ratio", f/16 most definitely being the latter. But I see this mistake thrown around a lot; it's good digital cameras are removing the need to think from photography. That way the Art can shine through.

    s-a
    I photograph things to see what things look like photographed.
    - Garry Winogrand

  2. #12

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    Quote Originally Posted by Yashinoff View Post
    And the F stop is a fraction of the focal length. f16 on a 500mm lens is a whole lot larger than f16 on a 50mm lens. It's the relation of the size of the hole to the wavelength of light going through it that causes diffraction.
    Really? I'll be damned. I always thought it was the ratio of light diffracted by the edges of the diaphragm to the undiffracted light coming through the clear aperture. I'll go back to physics 101 I guess.

  3. #13

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    I'm sure you know what I mean.

    (in case you didn't: that diffraction is correlated to the size of the aperture, and the f stop number is a fraction of the focal length of the lens.)

  4. #14
    ath
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    Quote Originally Posted by Yashinoff View Post
    It's the relation of the size of the hole to the wavelength of light going through it that causes diffraction.
    More precisely it causes a certain diffraction angle. The focal length transfers this angle in a linear blur and that's where the aperture (size/FL) comes into play again.
    Regards,
    Andreas

  5. #15

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    Quote Originally Posted by Yashinoff View Post
    I'm sure you know what I mean.

    (in case you didn't: that diffraction is correlated to the size of the aperture, and the f stop number is a fraction of the focal length of the lens.)


    Technical Note: Independence of Focal Length
    Since the physical size of an aperture is larger for telephoto lenses (f/4 is has a 50 mm diameter at 200 mm, but only 25 mm diameter at 100 mm), why doesn't the airy disk become smaller? This is because longer focal lengths also cause light to travel further before hitting the camera sensor -- thus increasing the distance over which the airy disk can continue to diverge. The competing effects of larger aperture and longer focal length therefore cancel, leaving only the f-number as being important (which describes focal length relative to aperture size).

  6. #16
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    The Cambridge In Colour site quoted above specifies that the f/number is relevant, not the absolute diametre of the diaphragm aperture. It says:

    Technical Note: Independence of Focal Length
    Since the physical size of an aperture is larger for telephoto lenses (f/4 is has a 50 mm diameter at 200 mm, but only 25 mm diameter at 100 mm), why doesn't the airy disk become smaller? This is because longer focal lengths also cause light to travel further before hitting the camera sensor -- thus increasing the distance over which the airy disk can continue to diverge. The competing effects of larger aperture and longer focal length therefore cancel, leaving only the f-number as being important (which describes focal length relative to aperture size).
    Fabrizio Ruggeri fine art photography site: http://fabrizio-ruggeri.artistwebsites.com
    Stock images at Imagebroker: http://www.imagebroker.com/#/search/ib_fbr

  7. #17
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    Quote Originally Posted by Yashinoff View Post
    I'm sure you know what I mean.

    (in case you didn't: that diffraction is correlated to the size of the aperture, and the f stop number is a fraction of the focal length of the lens.)
    So, that big chunk of light bending glass in front of the aperture makes no difference?

  8. #18

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    Quote Originally Posted by BradS View Post
    So, that big chunk of light bending glass in front of the aperture makes no difference?
    Less of a difference than the film format apparently, as per the information given in the link mentioned above. I guess I was a spouting a half truth, the resolution afforded by say a 4x5 format would make the diffraction from a small aperture like f45 basically negligible. However the same focal length/stop used on 35mm would suffer from ill effects of diffraction due to the smaller circle of confusion vs. the diameter of the airy disk. Play with the calculator it's interesting stuff.

  9. #19

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    "...the resolution afforded by say a 4x5 format would make the diffraction from a small aperture like f45 basically negligible. "

    Resolution has nothing to do with format size. F:45 on 4x5, using a 150mm lens is noticeably softer than f:22 with the same lens. The ratio of enlargement is usually much smaller with 4x5 though, (as well as shallower depth of field due to longer lenses) so smaller apertures are more commonly used on large format.

  10. #20

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    "This calculator shows a camera as being diffraction limited when the diameter of the airy disk exceeds what is typically resolvable in an 8x10 inch print viewed from one foot."

    With that qualification, the smaller the format the more it will be affected by diffraction. Now of course the diffraction won't actually be any different itself for any given format size, it just will be less noticeable for a given print size the larger the format is.

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