This is simply the angle of view with the binocular’s magnification figured in. For a 10X magnification and a 5-degree angle of view, the apparent angle of view would be 50 degrees.

The square of the exit pupil diameter. A binocular with a 4mm exit pupil has a brightness index of 16. A common way to measure the brightness of an instrument.

Binoculars using a central focus-control mechanism to allow both eyepieces to be adjusted simultaneously for quick focusing. Virtually all general-purpose binoculars now have this feature.

The ability of a binocular to focus closer than the “average” close distance of about 10 feet. A low close-focus number is useful when viewing small objects such as butterflies.

All modern binoculars have anti-reflection coatings on lenses and prisms, but multicoating and other complex coating methods reduce internal reflections and increase light transmission for a brighter view. Many companies claim their proprietary coasting systems enhance performance.

A separate adjuster on one eyepiece (usually the right) that allows the user to compensate for the difference between eyes. It’s a feature of all better-quality binoculars.

The point at which all the light rays passing through the binoculars exit through the eyepiece. To calculate the exit pupil, divide the diameter of the objective lens by the magnification. For example with an 8×40 binocular, the exit pupil is 40 over 8, which equals 5mm. A large exit pupil is important for low-light viewing.

The distance behind the ocular lens at which the image is projected to its focal point; it varies from about 5mm to 23mm. The greater the eye relief, the easier it is for eyeglass wearers to see the entire field.

The diameter of the circular viewing field seen through a binocular. It’s usually listed on the binocular, either in degrees or feet, measured at 1000 yards. One degree equals 52.5 feet over 1000 yards.

These have large objective (front) lenses that provide better light-gathering ability than a compact binocular with the same magnification, e.g. an 8×42 full-size binocular is much brighter than a compact 8×25.