Hello OwenM;
...I think collimation, or incomplete merging of the images, could cause CA. It may be a similar situation to being out of focus.
CA is an optical phenomenon (i.e., an aberration), whereas image "merging" is a psychological phenomenon done by the brain. The optics, in this case, include both the binoculars and the optics of the eye (exclusive of the retina) operating together as a coherently coupled system. It is known that the eye focuses at an intermediate frequency that minimizes defocus on either side; hence, a smaller axial chromatic spread will minimize blur and increase perceived sharpness. Note, however, that axial CA is a primary cue for visual accommodation, so it also serves an important purpose.
CA is at its most obvious when the IPD is off and/or focus is off. I am so used to focusing and looking through 30x instruments that at regular bino powers I probably would not notice CA even if others considered it to be terrible. I have a very good focusing technique so usually no problems there.
These are two different circumstances. When the IPD is set incorrectly the optical axes of the instrument and the eye are not aligned, leading to very complex aberration phenomena, including higher order CA. Assuming correct alignment, however, defocus within the chromatic range will effect clarity and measured resolution. Ironically, eliminating transverse CA may actually reduce grating acuity because "... The presence of color fringing my betray the presence of a gap between two bars even if the overall intensity modulation may be flat below the detectability threshold." (Mouroulis, pg. 173). So, even objective definitions such as "resolution" become problematic when the biological element is taken into account.
I am not particularly adept at IPD setting however. If I am testing optics, or going to use them for more than a few minutes, I set the IPD with a gauge of some sort.
My eyesight is not the best, about 20/25 with 0.25/0.5d of astigmatism and I do not wear glasses but I almost never see CA, but then, I do not go out of my way looking for it.
I find that a scale is necessary as well. Looking for CA invariably involves viewing off-axis, often at a large angle, whether or not birding tasks place such a demand on use of the instrument. So, I think the prudent approach is to take issue with the design only when color fringing actually interferes with appreciating the objects of attention.
All that being said, I am not sure whether some are just susceptible to CA and others immune, or if a good technique may help reduce the effects.
The individual variables are so numerous, such as IPD setting, eyeglass correction, instrument collimation, eye movement strategy, selective attention, etc. that I judge it to be near impossible to determine whether or not some people really are more "susceptible" or sensitive than others to CA. Although it is often claimed, as a scientific matter it would be very hard to verify.
Ed
PS. My motivation for mentioning most of the above is to point out that: (a) the retinal image is a joint function of the optics of the instrument and the eye, and (b) the behavior of the eye is a function of the retinal image. The perception of CA is, on the one hand, the
conscious experience of color fringing, but, on the other hand, it is also the
unconscious effect on visual performance. To my knowledge, optimization criteria are not easily established, since these two effects are somewhat at odds with one another.
Next time I go to the beach I´ll bring porros too just to compare. Have a great week!
