EXCERPT page containing first few paragraphs. 2020-10-26 12:43:42
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The example shown here is with a Nikon D800E given special attention per my request to Nikon service following my detection of some minor sensor-to-flange alignment hints that I saw with several lenses
It is not uncommon for even a well made lens to have a slight left/right asymmetry across the frame, especially as focus changes; those elements have to move and they can never be assembled absolutely perfectly nor can they rotate and move in absolutely perfect planar fashion down to the last micron of precision. Lenses with manual focus helicoids and autofocus lenses all show the issue; see for example the Sigma 120-300mm example. Furthermore, lens elements have to be ground to some degree of precision, and small variations mean that there is always natural variation here and there, especially as those elements are rotated. Perhaps only the ultra expensive cine lenses avoid the issue (presumably), by virtue of exacting quality control standards.
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