How Binoculars Work

A key thing to know about binoculars is they’re never going to let you see farther than you can with your naked eye. Instead, binoculars bring objects closer so that you can observe and resolve details you otherwise could not. Thanks to binoculars, a flock of birds suddenly becomes separate, distinct birds. Stars appear to zoom closer as if in a cool outer space movie. A faraway animal is easily identifiable, down to the minute detail.

Many people think they own a pair of binoculars, perhaps Dad’s old set, but what they really have is sports glasses, opera glasses or theater glasses. They do not have prisms that magnify the view; instead, there is simply an eye lens and an objective lens lined up straight. The magnification power is low; generally 2x to 4x. Slightly better are field glasses, which are constructed the same way, but may have a power level as high as 7x. All of these are known as nonprismatic glasses. They’re affordable and quite suitable for viewing things at close range, such as sporting events or theater productions. There’s nothing illegal about calling such glasses binoculars, but it is somewhat misleading.

Binoculars, on the other hand, are essentially two identical scopes, each of which has two lenses capable of gathering and enlarging an image. Because there are two sets of lenses, the image we see has depth. The prisms (positioned between the objective lens and the eyepiece) adjust for the refracted light and turn the image right side up for the viewer.

True binoculars are comprised of two prisms (or a prism plus a mirror) as well as three or more lenses: an objective lens, a field lens and a positive ocular lens.

The Porro prism design includes an objective lens, two triangle-shaped Porro prisms, one or more field lenses and an ocular lens. The distance between the objective lenses is greater than the distance between the eyepieces. Porro prisms used to dominate the market, but now it’s roof prism models that are more prevalent and considered by most to be of higher quality.

The roof prism model sports convex lenses at the outer surface, as opposed to field glasses in which the lenses are concave. Inside, there are two prisms but they are attached back-to-back and thus light is treated differently; it’s generally accepted that roof prisms transmit light better than Porro prisms. Roof prism models are also typically more compact.

You may also see specific reference to the types of prisms used. BAK4 prisms, made of barium crown glass, are considered to be top-quality. BAK-7 prisms, made of borosilicate glass, are also acceptable, though with a little less “brightness,” as the glass is less dense. (Some users avoid binoculars made with BAK-7 prisms.)

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