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ronharper
Pooh-Bah
Reged: 02/14/06
Posts: 1276
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Re: ED glass in binoculars
07/02/09 12:59 AM
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Ed,
That is a great summary of the astro scope issue, longitudinal CA. I believe this is the effect that puts a haze of poorly focused blue and violet around a bright object. I have seen this in my achromat scope, and also in binoculars, mostly around Venus in my 16x70. It rarely seems to be a problem in binoculars.
But in the daytime, in common high contrast situations, like a bird sitting high in a tree against a bright cloud, I often see color fringing that gets worse the farther from the center of the field you go, and is yellow-green on one side of the object, and purple on the other. That is annoying. That's a different kind of CA, the lateral kind.
The origin of lateral CA is not clear to me. Some people blame the eyepieces, and that can clearly be a culprit, because Nagler type eyepieces show lateral color even with a mirror objective. Also, if the exit pupil is partly occluded by an off center eye, an achromatic objective will contribute to the effect. Whatever the origin, that is the kind of CA that bugs me, and I think most people, in binoculars. And whatever the explantion, most people report that lateral CA is significantly reduced in ED glass binoculars. Some critical observers have gone so far as to say that, for example, the Zeiss FL series completely eliminates this effect over the central half of the field of view!
I am apparently rather sensitive to color fringing, and blessed be those who aren't. In high contrast situations always I am careful to center my eye, and the object. Good technique helps, but doesn't always work. This morning I looked at a bird in a tree in such a bad situation with my 8x42 Trinovid BA. I just couldn't get rid of the color fringing. The view was confused, yet I thought I could just see a trace of reddish color on his head. Then he flew to a better visible location, and turned out to be a brightly colored Western Tanager. The bird's yellow was very similar to the yellow green color of the fringes. I can't help but wondering if a Zeiss FL, or similar, would have solved that problem.
Ron
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