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Help understanding diffraction

Discussion in 'Nikon DX DSLR Forum' started by freundez, Sep 29, 2008.

  1. Hi all,

    I'm hoping to understand a bit more around the subject of diffraction and the role that sensor size and pixel density plays in diffraction at smaller f/stops (f/16, f/22, etc.).

    I was reading an article in Outdoor Photographer that contained the following excerpt:
    A related full-frame-sensor benefit is less diffraction. When you stop a lens down to increase depth of field, diffraction starts to affect image quality adversely. The smaller the lens opening, the greater the diffraction, and the worse the image quality. Because they produce a given angle of view with a longer lens, full-frame sensors yield less diffraction at any given ƒ-stop: A 24mm lens stopped down to ƒ/22 has an aperture diameter of 24/22 or 1.09mm. The 16mm lens needed to produce the same angle of view with an APS-C sensor has an aperture diameter of just 0.72mm at ƒ/22 (16/22). Of course, the shorter lens produces more depth of field than the longer one, so you don’t have to stop down as much to get the same depth of field. But the super-short focal lengths also tend to produce more distortion and vignetting.

    While it makes sense to me that a wider angle lens would have a smaller effective diameter at a given aperture, I was under the impression that a lot more contributes to diffraction effects than this. Can anyone explain (or direct me to a decent article that explains) the relationship, if any, between lens, sensor size, and pixel density on diffraction?

    I'm also curious how big an issue this is in real-life - if anyone has examples that demonstrate the value of a full-frame sensor vs. an APC sensor relative to diffraction I'd be interested in seeing them.
  2. fks


    Apr 30, 2005
    sf bay area
    hi eric-

    i found this linkon dpreview. i haven't read it yet as it's quite detailed.

  3. Thanks - I hadn't seen that article. Makes a lot more sense now.
  4. garyosborne

    garyosborne Guest

    well i think you already have the gist of it pretty much. FF has approx 1 stop advantage over APS with regards diffraction. APS has approx 1 stop advantage over FF with regards to DOF. I think using apertures of F16 and smaller is the realm of macro, therefore, they sort of cancel each other out. For landscape if you focus at the hyperfocal length small apertures may only be needed in special situations. This may be too simpified explanation. I shoot macro and never use more than F11 on D300 because I want the sharpest pictures possible
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