Not to detract from what Joseph has written, which adds some subjective
detail missing from the following, but to add the foundation to it as a
reference. There is a basic tutorial about diffraction limits on Phil
Greenspun's "photo.net" site:
http://www.photo.net/photo/optics/lensTutorial.html
See "Diffraction," the 5th topic down the page.
-- John
At 05:22 1/31/00 , Joseph wrote:
>
>Some people sent me email to ask about my previous posting about
>diffraction limits of lenses, and I thought I would add the following:
>
>First, a precisely calculated value can only be obtained at a particular
>focal distance. When light passes through an aperture, diffraction
>takes place, and the light starts spreading out. The smaller the
>diameter of the aperture the faster the *rate* at which it spreads
>out, and so the further it travels after that the worse the effect.
>
>What this means is that at close focus, a lens is farther from the
>film than at infinity focus, so at a given aperture, the diffraction
>limit of a lens is a lower resolution value at close focus than
>at infinity focus. Thus, you would have to calculate a precise
>range of diffraction limits for a given aperture not one precise
>value.
>
>Second, this effect is why teleconvertors degrade macro shot images
>less than landscapes. John Shaw, in his book, "Closeups in Nature"
>notes that teleconvertors have a lower impact on quality of a
>lens in macro situations but does not give the reason.
>The reason is that in macro situations, the lens is further
>from the film plane, and the lens is usually well stopped
>down for depth of field. These effects result in a lower diffraction
>limit for the lens, and the loss of resolution due to the teleconvertor
>is bounded by the difference between actual resolution of lens + TC
>and the diffraction limit. This is because even a perfect lens could
>not do better than the diffraction limit, so a lower diffraction limit
>on the aerial resolution of a lens in macro situations translates to
>a tighter bound on the loss of quality due to teleconvertor.
>
>Cheers,
>
>Joseph
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