Hold the binoculars a foot away and look for a bluish or purple tinge to the lens. You should be able to bring the barrels close enough together so that the images merge into one, single, clear image within a perfect circle, Improper binoculars can cause eye strain and headaches. If you wear glasses, make sure the rubber cups around the eyepieces fold back to accommodate eyeglasses. Get binoculars that produce a clear image of an object as close as 20 feet,or you may miss some birds. Look for minimal image distortion at the edges of the lens.
Chromatism is the rainbow-colored tinge around an object seen through a lens.
Bad binoculars will create this effect, especially near the edge of the lens. Good binoculars will have very little chromatism. When using binoculars, learn to look first for easily identifiable characteristics of the bird's immediate area in order to locate the bird. For instance, observe if the branch your bird is perched on is crooked, broken, short, or v-shaped. These kinds of observations will greatly help you to then spot the bird through the binoculars.