Magnification or Power- Binoculars are often referred to in two numbers, with the first number separated by an “x”. The first number is the power of the binocular. In an example of 7 x 50, an object would appear seven times closer with this binocular.
Objective lens- This is the second number of a binocular and refers to the size of the front lens as expressed in millimeters. The bigger the objective lens (second number of “8 x 42”), the more light or image detail will be captured. Compared to the naked eye, even an 8 x 21 compact will capture much more image detail.
Zoom lens- A binocular of variable power
Eye relief- The distance that a binocular can be held away from the eye and still show the entire field of view. The longer the eye relief, the more comfortable the binocular is to use. This is great for eyeglass wearers whose glasses limit how close they can get to the eyepiece.
Field of view- The side-to-side measurement of the maximum viewing area expressed in feet. This is usually measured at 1000 yards or meters.
Close focus- The minimum distance that a binocular will focus. For example, a close focus of 10 feet means you can look at something only 10 feet away and still be able to focus the binoculars. This is especially important for birding or butterfly watching.
Resolution- The ability of an optic system to distinguish fine detail or retain clarity.
Prisms- Special lenses that reduce the size of an optical system. They reverse the images to make viewing easy.
Roof prisms- Prisms that are overlapped closely to form a narrow thin shaped binocular. Found mainly in lightweight and compact designs. A narrow field of view is typical.
Porro prism- Prisms that are offset from the eyepiece. Generally larger binoculars, these offer superior depth perception and a wide field of view.
Prism glass- Most binoculars are made with BK-7 glass. Better models feature BAK-4 glass which gives better sharpness and less distortion.
Focusing systems- Insta focus, a paddle or rocker to adjust focus; Center focus, a wheel between the barrels; Perma focus, no focusing required, pre-set.
Exit pupil- Refers to the column of light at the back of the eyepiece. The greater the circle, the more light that gets to the eye. This is calculated by dividing the power number into the objective lens number. For example, and 8 x 21 has an exit pupil of 2.625, while a 7 x 50 binocular has an exit pupil of 7.15. Therefore, the 7 x 50 will be a brighter binocular.
Optical or lens coatings- all lenses tend to reflect rather than capture light. Lens coatings reduce or eliminate reflections and increase light capturing capability. Most binoculars today have some kind of lens coating applied during manufacture; however, degrees of coverage vary. The more level of coatings, the better the binocular. There are four levels of coatings: 1) coated, single-layer, 2) fully-coated, single layer on all glass surfaces, 3) multi-coat, multiple layers on at least one lens surface, 4) fully multi-coated, multiple layers on all surfaces.