My favorite way to
test a
binocular for optics is under the stars. Just how good are your favorite
birding binoculars or
astronomy binoculars? First, adjust your
binocular, carefully, by sighting on some stars. Point sources of light are perfect for getting your diopter adjustment just right, anyway. Then, look at the moon, specifically at the edge of the moon. A binocular with good color correction will show only a trace of color fringing at the edge of the moon. A binocular with poor color correction will show an annoying amount of color fringing. Then, look at a planet, such as Jupiter. Do you see a tiny disc, free of spikes or flares? Takes a pretty good lens to turn Jupiter into a perfect little disc. Mars, due to its smaller size, is even more of a challenge. An average binocular will only show it as a small blur of light, but an excellent binocular will show it as an ever so tiny disc. How does your binocular test?