I don't think binocular length plays a significant role in controlling CA. Focal length would need to increase ~4x to have mitigating affects. More likely, a SLIGHTLY longer focal length helps mitigate the other image damaging aberrations in a fast refracting lens system, spherical aberration and astigmatism.
Aperture and magnification are much more relevant factors for the kind longitudinal CA and spherochomatism that plagues fast refractive optics. Larger apertures and higher magnifications increase the chance longitudinal CA will be visible. Longitudinal CA is best recognized by objects in the unfocused foreground and background showing green and purple edges. The colors do not swap positions when you tilt the binocular or look around the FoV. Longitudinal CA at sub-15x magnifications of focused objects in the center of the FoV may not be visible (though it is still there albeit ever so slightly). This is why a well made non-ED binocular can still give nice views.
Lateral CA, the type most likely seen and reported on with handheld binoculars, is determined by eye position, exit pupil size and the AFoV. It is best recognized by the purple and green outlines of high contrast, clearly defined objects swapping positions as you tilt the binocular or move you eye around the FoV.
It is important to remember that neither longitudinal or lateral color occur in isolation and most lens systems exhibit both simultaneously. Better control of all types of chromatic errors is a benefit of ED glass (due to the extra degrees of freedom it allows in the figure) and why images can be slightly sharper with better contrast and saturation, i.e. "pop", when ED glass is used as more of the white light spectrum is in focus across the FoV.
Well said! There you go, TP. Your answer in layman's terms. You just need to use your Little Orphan Annie Decoder Ring.
What I find interesting is that sometimes what is "supposed to be" - like the 4x FL rule for reducing CA - doesn't hold up in real world situations even though it might look good on paper. In every instance where I've tried an 8x32 and 8x42 in the same series of (non-ED) bins, the 8x32 showed more CA.
Same difference with two 8x midsized bins with different FLs. The 8x30 EII shows more CA than the 8x32 SE even though they are mooreorless the same configuration.
Here's what Henry had to say about this (I think he had the 4x FL rule in back of his mind when he wrote this but his eyes were telling him differently, just a guess):
"The optical trade offs (of the 8x30 EII) compared to the SE are reduced eye relief from the shorter focal length eyepiece needed to produce the same magnification and higher levels of chromatic and spherical aberrations from the lower focal ratio objective (which may or may not actually be visible at 8X)."
Oh, it is, Henry, it is. At least for me it is. I very much like the 8x30 EII despite it showing more CA than the 8x32 SE in high contrast situations. But when viewing a bird against a bright, cloudy sky, the ZR 7x36 EDII does a better job than both of them in reducing CA to a minimum, with its double blow of lower magnification and ED glass. OTOH, it has more pincushion than both of the Nikons put together.
Like Roseanne Roseannadanna used to say: Well, it just goes to show you, it's always something--if it ain't one thing, it's another.
Here's Henry's in-depth review of the 8x30 EII from which I excerpted the above quote:
http://www.birdforum.net/archive/index.php/t-38202
Brock
