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Message: <blockquote data-quote="elkcub" data-source="post: 1323923" data-attributes="member: 14473">Ron,It seems to me from what you said the Stokes test reveals the extent of underlying longitudinal (axial) CA, assuming that lateral CA has been corrected by the achromatic design of the objective.  Apparent color fringing is therefore enhanced by covering the lower half of the objective because the "... unfocused light below the chosen focus point is now all yellow green, and as a result, stands out glaringly, as color fringes. The fringe above the focal point is purely made of red and blue, so looks purple..."  Correct me if I'm not getting this right.  Covering half of an achromatic objective should produce more apparent fringing, not less.  A related implication is that whether seen or not as color fringing, axial CA still degrades the image.  Regarding your interesting comments about eye-ocular interface effects, masking  by the iris might contribute an increase in "apparent" CA (i.e., due to the Stokes effect), but the aggregate aberrations of off-axis or cross-axis viewing have never been clear, at least to me.  It's not an unknowable subject, but it would no doubt take extensive computer modeling for a particular binocular design.  Simplifying equations are needed, but so far I've not seen even a rough approximation to how CA (both types) produced by the objective, prism, eyepiece, and eye combine in the final retinal image.  The nature of CA suggests that net effect involves cancellation as well enhancement.  Thanks for the provocation to think.  Ed</blockquote>

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