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Old Aug 14, 2009, 10:27 AM   #4
Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378

Because aperture as expressed as f/stop is designed to express a ratio between the focal length of the lens and the size of the aperture iris, you have to take focal length into consideration when computing diffraction limitations. ;-)

The article that Mark1616 linked to has a calculator for that purpose.

But, in addition to diffraction related issues, you also have to take each lens on a case by case basis in order to determine the optimum aperture for a given focal length for sharpness/contrast purposes.
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