When you buy binoculars you should check their performance at the store, or if you buy them online, as soon as they arrive. Here are a few tests that will help you choose a quality binocular.
1. Look carefully at the objective lenses. Reject the binocular if you see signs of scratches or dirt.
2. Consider the weight and bulk of the instrument; lightweight binoculars are less tiring to hold than heavier ones.
3. Is the instrument soundly made? Pick it up and move the halves back and forth. The hinge should work smoothly, with steady resistance. If there is any play in the joints or anything rattles put it back and try another.
4. Hold the binoculars at arms length away from you, and point them at the sky or a window. The exit pupils should be truly circular and uniformly bright.
5. For most binoculars collimation problems are not immediately obvious when you first pick the instrument up and view through it. If after using the binoculars for several minutes your eyes feel uncomfortable as they compensate for the barrel misalignment, most probably the binoculars are out of collimation, which means that the two barrels don’t point in the same direction. This is a serious problem, and you shouldn’t buy those binoculars.
6. Look through the binoculars, find a high-contrast object such as a tree against the daytime sky, and place it in the center of the field. If you see strong green or violet fringes of light along the edges of the object, the binoculars suffer form a defect known as chromatic aberration. In general, the higher the magnification, the bigger the chromatic aberration. This error is not critical for astronomical use, but it can be a real problem for birders and other nature enthusiasts that want accurate color rendition.
7. Check the type of antireflection coatings by looking at the two reflections of the light from the front and back of the objective lenses. Multicoated lenses usually give green or purple reflections, while simple coated lenses generally have blue reflections. Don’t buy binoculars that give bright white reflections; they have no antireflection coatings and will perform poorly for astronomical use.
8. If you choose binoculars with large fields of view, you might notice that the edges of the field are conspicuously distorted. This optical defect is called distortion, and is most obvious during daytime use. Even high quality binoculars have some degree of distortion, but it hardly noticeable, especially in astronomical viewing.
9. The ultimate test for your binoculars is to take them outside at night and look at a bright star. After you have the star centered in the field of view and bring the binoculars in good focus, take a close look at it. The star should be a sharp near-pointlike imagery, without any irregular spikes or rays shooting out of it. Small deviations from this ideal image are always present to some degree in most binoculars, and this is perfectly OK as long as the star remains a round disk, even when moved towards the edge of the field.
One last word: Remember that you always get what you pay for. In most cases the brand name tells a lot about the quality of the instrument, so avoid buying form companies with which you are unfamiliar. Brands like Nikon, Minolta, Leica, Leupold, Bausch and Lomb, Tasco, and Bushnell have a reputation for high quality standards, even for the inexpensive models.