Binoculars, as opposed to monoculars or telescopes, allow you to see an object in three dimensions. By focusing on an object through two lenses, at slightly different angles, you get depth perception. You also get the added convenience of looking through both eyes, as opposed to closing or covering one eye.
When selecting a pair of binoculars you must take into consideration the primary purpose for which they will be used. Binoculars are designated by ‘magnification x objective lens diameter’ and field of view. For example: 7 x 50, 372 ft. at 1000 yds., would indicate a magnifying power of factor 7 and objective lenses with a 50 mm diameter, and a width of view 372 ft wide, 1000 yards away.
So what does that mean? The magnification factor of 7 indicates that the image you are seeing is as if you were 7 times closer to the object. The diameter of the objective lens determines how much light can be gathered to form the image. And, in this case, a field of view of 372 ft. at 1000 yds, would indicate that if you were looking at a football field from 1000 yards away, the entire field, including the end zones (360 ft) would nearly fill your field of vision.
Although there are some bench mount models available, most binoculars are small and light in order to control shake in the field. There are trade offs, though. If you’re primarily working in low light situations, you’ll want a set with larger objective lenses to let in more light. Larger objective lenses would add more weight. If you’re looking for greater magnification, you’ll want a set with a longer objective lens focal length, which means longer barrels.
There are a couple more things to be aware of. Many binoculars have an anti reflective coating on the lenses. Some have it on one surface of one lens per barrel, some have it on all lens surfaces, and other may be anywhere in between.
Eye relief – this determines the distance the user must place the binoculars from their eyes in order to see an unvignetted image. The longer the focal length of the eye piece lens the greater the eye relief. Greater eye relief is important for people wearing glasses.
Exit pupil – this is the beam of light gathered by the objective. Its diameter is the objective lens diameter divided by the magnifying power. For maximum effective light-gathering and brightest image, the exit pupil should be about 7 mm, the diameter of the fully dilated iris of the human eye. Light gathered by a larger exit pupil is wasted. For daytime use an exit pupil of 3 mm, the eye's contracted pupil size, is sufficient. However, a larger exit pupil makes alignment of the eye easier and avoids dark vignetting intruding from the edges.
If you will be using the binoculars for viewing at varying distances, you’ll also want to consider the ease of focusing. Some binoculars have independent focusing where there is an independent focus at each eyepiece. Central focusing has a central adjustment that adjusts both barrels with one wheel and an adjustable eyepiece in one of the barrels to allow for differences in the observer’s eyes.
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