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Message: <blockquote data-quote="henry link" data-source="post: 3194851" data-attributes="member: 6806">A flat field means that field curvature (and astigmatism) have been corrected so that objects are sharp across the field, but that can be accompanied by any amount of rectilinear and/or angular magnification distortion. Zoom eyepieces run the gamut from barrel distortion to pincushion with varying amounts of AMD present, usually changing with the magnification setting. I haven't seen all of the newest zooms, but I recently evaluated the Kowa 25-60 and found it to have barrel distortion at all magnifications, with the inevitable AMD that goes with that.Distortion is not noticeable in most simple "planetary" eyepieces, like Zeiss Abbes, TMB Super Monocentrics, Brandons, Claves, etc. because the fields are narrow enough to remain nearly orthoscopic. On the other hand, some wide field astronomical  eyepieces, Like Tele Vue Panoptics and Naglers have such huge amounts of pincushion distortion that they create reversed AMD, so that objects near the field edge are stretched radially rather than compressed.What I mean when I say that most modern zooms are as good as fixed magnification eyepieces is that they don't degrade the axial performance of even the most highly corrected scopes. The only exception would be in the areas of light transmission and contrast, where some very simple eyepiece designs with only one or two lens groups are slightly better. In those performance areas the best current zooms compare favorably to other equally complex wide field fixed focal length eyepieces.</blockquote>

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