Diffraction Limited Optics

greenspun.com : LUSENET : Pentax 67 SLR : One Thread

Most of us have probably heard this term used by optics producers, usually when they are talking about the quality of their products. This term is used way to loosely by the optics industry. It is similar to the over use of the term APO. It leads people to think that their lenses are so well corrected that the only cause for unsharpness is diffraction. This is not possible. Although at small stops diffraction will generally be the most dominant cause for unsharpness, it is not the cause for unsharpness at wide open apertures. There is a point for every lens, when stopping down, that normal aberrations will diminish and diffraction starts to dominate. This is the stop that is considered the sharpest for that lens. For any producer of photographic optics to claim that their lenses are so good that only diffraction limits the sharpness, is being unrealistic. SR

-- Steve Rasmussen (srasmuss@flash.net), March 06, 2001

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