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Old Aug 14, 2009, 10:33 AM   #6
Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378

I'm wrong. Focal Length doesn't matter. You'll see this section at the bottom explaining why:

Since the physical size of the lens aperture is larger for telephoto lenses (f/22 is a larger aperture at 200 mm than at 50 mm), why doesn't the size of the airy disk vary with focal length? This is because the distance to the focal plane also increases with focal length, and so the airy disk diverges more over this greater distance. As a result, the two effects of physical aperture size and focal length cancel out.
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