Exit Pupil (EP): Indicates the amount of light that enters the user’s eyes. EP is computed by dividing the diameter of the objective lens by the magnification, (e.g., an 8×40 binocular has 5mm exit pupils). In bright light, an exit pupil of 2.5mm is sufficient. For all-around use, 4mm is acceptable, with 5mm or 6mm being better. For night use, 7mm is best.
Antireflection Coatings: These coatings greatly reduce the light- scattering reflections that occur at air-to-glass surfaces. The results are improved image contrast and light transmission.
Specification Numbers: The number preceding the “X” is the “magnification” or “power” (e.g., an 8×40 binocular makes objects appear eight times larger). For general use, 7X to 10X binoculars are best. The number following the “X” is the diameter of the “objective” (front) lenses in millimeters. In general, larger-objective binoculars produce sharper images and admit more light, but they will be heavier.
Eye Relief: This is the distance from the ocular lens to the point where the eye must be positioned to see the full field of view. (Note: Eyeglass wearers need between 15mm and 25mm of eye relief to see the entire field of view.)
Eyecups: Retractable eyecups that twist or slide in and out are rapidly replacing traditional fold-down eyecups. Retractable eyecups are fine, but only if they don’t retract too easily.
Prism Type: Binoculars are distinguished by their prism type: With Porro prisms (like those shown here), the objective lenses are offset from the eyepieces; with roof-prism models, the objective lenses and eyepieces are in straight alignment. Either type can be excellent, though Porro prisms tend to be less expensive.