What does "anastigmat" mean

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Can anyone tell me what the term anastigmat means as applied to a large format lens? Is this a good thing (particularly in a long lens to be used for detail in landscapes)?

Thanks in advance, Mark

-- Mark DeMulder (mdemulde@usgs.gov), December 11, 1999


"Astigmatism" is an optical aberration wherein the tangential and sagittal axes of a lens focus on different planes. Consider the circle of coverage of a lens. Now imagine a point near the edge. If you draw a line from that point to the center, then that's the sagittal axis. A line perpendicular to the one you've just drawn would be the tangential axis. In a lens with a lot of astigmatism, you can focus sharply on modulations along the sagittal axis, or on modulations along the tangential axis, but not on both at the same time. In practice, unless the astigmatism is REALLY bad you can usually work around it by stopping down to small apertures.

An "anastigmat" is simply a lens which is corrected for astigmatism. Almost all modern LF lenses are pretty well corrected for astigmatism, though many older ones aren't. Back when some were and some weren't, the makers differentiated the ones which were by calling them anastigmats.

-- Patrick Chase (patrick@sdd.hp.com), December 11, 1999.

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