Cameras and Lenses

Is this "CA"?

I´ve never noticed "CA" before, and am not sure what it is. I took the kids to the beach today, and brought a pair of premium bins "just in case". While looking at the horizon, against a bright sky, I noticed there was no "fringing" if the horizon was centred in the viewing "circle". But if I tilted the bins downwards, so that the horizon was in the upper fifth section of the circle, there was a pinkish-purplish fringe along the skyward side of the horizon. If I tilted the bins upwards, so that the horizon was in the bottom two-fifths of the circle, there was a bluish-green fringe to it. I then tried the same trick with flying cormorants, and found, amazingly enough, the following: with the cormorant centred, no fringe. With the cormorant in the upper fifth, a purplish fringe on the underside of the bird. With the cormorant in the lower two-fifths, a greenish fringe on the upperside of the bird. None of this bothers me, but I´m curious to know what it is. Needless to say, I didn´t see any amazing birds, otherwise I wouldn´t have been playing "find-the-fringe" with my binoculars. But is that "CA", or is it something else? Thanks in advance.

Sancho,

What you noticed is lateral chromatic aberration. I'm convinced that when people complain about "CA" in binoculars that's the form of chromatic aberration they are always talking about. The other form, longitudinal chromatic aberration, is essentially invisible as a color fringe at the low magnification of binoculars. Lateral CA should be absent from the center and grow more pronounced toward the field edge, but I've found that it becomes visible quite close to the center in some binoculars and is sometimes even visible directly on axis if your pupil is slightly decentered. Oddly enough, I think it is a far bigger problem in expensive binoculars with complex objectives and internal focusing lenses than it is in cheap simple Porros.

Henry

Sincerest thanks for that, Henry...now I can speak knowledgeably about "Lateral CA"! Next time I go to the beach I´ll bring porros too just to compare. Have a great week!

Back to Home Page!