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binoculars

power
aperture
field width
exit pupil
eye relief

Manufacturers do not provide figures of merit for some of the most important characteristics of a binocular; above all, the tolerance permitted in aligning the two halves. Collimation is critical, but there is no easy way for the consumer to check it. Other features, such as the quality of the lens coating and the effectiveness of internal baffling, can also be important.

Power

The figure before the “×” is the power: the amount things will appear to be magnified. With a pair of either 7 × 50 or 7 × 35 binoculars, for example, things 1000 feet away would appear as large as they would if the viewer were standing (1000 divided by 7 = ) 143 feet away.

At magnifications greater than about 8 or 9 power, normal shaking of the hands may make the image so unsteady that it becomes necessary to support the binoculars on a tripod.

Aperture

drawing of binoculars, pointing out lens in the front

The number following the “×” is the diameter of the objective lenses in millimeters. The larger the aperture, the greater the light-gathering power. Other factors, however, may prevent some of this light from reaching the viewer's retina.

Field width

Sometimes this is given in degrees, sometimes as a field of view: “300 feet at 1,000 yards” or “120 meters at 1,000 meters.”

To compare such specifications, enter a number in any of these boxes, and click outside them.

The optical design of some binoculars gives them a wider-than-usual field of view.

Exit pupil

The diameter of the exit pupil is sometimes given; if not, it can be found by dividing the aperture by the power. Thus the exit pupil of a pair of 7 × 50 binoculars is 50 divided by 7 = 7.1 mm. The bigger the exit pupil, the less critical the alignment of the binocular and the eye.

In dim light, another effect of the size of the exit pupil becomes more important. Assuming two binoculars are the same power, the one with the bigger aperture will have the bigger exit pupil. As the size of the exit pupil increases, the image appears brighter, up to the point at which the exit pupil is the same diameter as the pupil of the eye and the entire area of the pupil is illuminated. After that, further increases produce no gain in brightness.

As people age the maximum opening of the pupil diminishes. The pupil of a healthy young person's eye has a diameter of about 2 mm in bright light, 5 mm in dim, and 7 mm in the dark. People in their thirties typically have a maximum pupil diameter of about 6 mm, which shrinks to 4.5 to 5 mm in their forties. (See the chart.) Thus, at dusk, people in their twenties would see a brighter image through 7 × 50 (exit pupil dia. 7 mm) binoculars than through 7 × 35 (exit pupil dia. 5 mm) binoculars, but persons in their sixties would perceive no difference.

Some manufacturers give a “relative brightness index,” found by squaring the size of the exit pupil in millimeters. The highest number allowed in this index is 49, since human pupils open no more than about 7 mm.

Eye relief

Eye relief is the distance between the eyepiece and the point of focus of the exit pupil. Currently marketed binoculars have eye reliefs ranging from 1 mm to 23 mm. The former is outlandishly small; eyelashes will dirty the lens. Large eye reliefs–around 15 mm is enough–are needed by persons who wear eyeglasses while using binoculars. Although the focusing adjustments of binoculars can compensate for most near- and farsightedness, they cannot compensate for astigmatism. Many who aren't astigmatic prefer to wear their glasses while using binoculars, simply because they don't like putting them on and off.