Okay, lemme see if I grok astigmatism correctly...
My CPC lens (and Canonet G-III QL17) exhibit what I call "cross-eyed" bokeh.
Rather than softly merging indistinct lines, there is a pattern of doubled
off-center and slightly overlapping patterns. For example, out-of-focus
leaves and specular highlights which produce the familiar iris-shaped
pattern in most lenses will produce two or more overlapping but off-center
iris-shaped patterns in my CPC lens.
That may be an indication of astigmatism, yes?
So if this is due to a flawed element rotating around another element during
focusing (or vice versa) then I should be able to see a pattern
demonstrating a change in the overlapping as focusing changes.
IOW, if the overlapping iris-shaped artifact is at 3-o'-clock at a
particular distance, by turning the focus ring this overlapping artifact may
shift to, say, 7-o'-clock. Have I grasped this correctly? Hope so, because
the concept of astigmatism has always eluded me, both in camera lenses and
my own eyeballs.
And its effect on critical focusing would explain something I've noted in
this CPC lens, that certain subjects have resulted in satisfactorily sharp
images while others have been disappointing. It may be due to whether I've
used the split-image or matte portion of the focusing screen.
Lex
===
Date: Mon, 15 Jan 2001 18:11:14 +0000
From: "John A. Lind" <jlind@xxxxxxxxxxx>
...It's my educated guess your lens has astigmatism. The best way to
describe it is lines along one axis having a different focus point than
lines along a different axis such as horizontal versus vertical. If memory
serves me correctly, a lens with a cylindrical component in its shape will
exhibit astigmatism. If you were to take a cross-section from the axis in
the center to the edge, and rotate it around the center, the cross-section
would change shape. Astigmatism in a camera lens will make it difficult to
critically focus, except perhaps by using a split image.
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