Comprehensive Glossary of Binocular Terms

Everything you've ever needed to know about binoculars, located in one handy place.

Aperture - The size of the binoculars' objective lenses. The second number represented in the numbers describing a set of binoculars, measured in millimeters. Example: In the Steiner 8x22 Predator Binoculars, 22 would represent the aperture.

Aspherical Lens - A lens with flattened edges, useful for a clearer, sharper image.

BaK-4 Glass - Premium, high-density barium crown glass that minimizes light scattering inside the binocular tubes and allows bright, crisp, clear viewing.

Center-Focus - A mechanism that allows both eyepieces to be adjusted at the same time, useful for rapid focus.

Central Focusing Wheel - A wheel mounted in the middle of the binoculars, allowing for focusing.

Coated/Multi-Coated Glass - Thin layer(s) of coating added to the binocular glass to help reduce reflections. Examples: (C) Coated optics - one or more glass surface is coated. (FC) Fully coated optics - all glass surfaces that have any vulnerability to air are coated. (MC) Multi-layer coated - one or more glass surfaces is coated multiple times. (FMC) Fully Multi-Coated - all glass surfaces susceptible to air are multi-coated.

Compass Binoculars - Binoculars with a compass built right in - perfect for finding your way back to the campsite after a long day of bird-watching or hunting.

Depth of Field - The portion of an image, between the foreground and background of the viewing area, that is in focus.

Digital Camera Binoculars - Binoculars with a digital camera built right in - useful for taking clear, magnified pictures of the illustrious 30-point buck, the rare and beautiful Chinese Crested Tern, or who knows - even Bigfoot!

Diopter Adjuster - A separate eyepiece-focusing tool, usually on the right lens, that allows the user to adjust the lenses separately to allow for eyesight differences.

Exit pupil - The amount of light rays that enter the objective lens and exit the ocular lens. The measurement is achieved by dividing the lens aperture by the magnification. Example: In the Nikon 10x50 Action Binoculars, the exit pupil would be found by dividing the aperture (50) by the magnification (10), equaling 5. A higher exit pupil means the binoculars will work efficiently in dim light. For well-lit surroundings, an exit pupil of 2.5 to 4 is sufficient. If you hold a pair of binoculars at arm's length, you'll be able to clearly see the circle of light in the eyepieces, representing the exit pupil.

Eye Relief - The actual distance light travels from the ocular lens to the eye, measured in millimeters. A typical range of eye relief is 8-13 mm. Eyeglass wearers especially will want to take note of this measurement; if the measurement doesn't allow for glasses, the user won't be able to see the whole field of view.

Eyecups - Cups on the eyepieces of binoculars that allow for positioning of the eyes and provide optimal eye relief. Some eyecups come in a rubber version that the user can fold down to accommodate eyeglasses. Other binoculars use cups such as 'twist-up' or 'pop-and-lock' that are more adjustable for any user.

Field Glass - A type of binocular that uses a second lens (instead of a set of prisms) to magnify an object. Field glasses are more durable than prism binoculars, although the magnification strength tops out at about 5x.

Field of View - The size of the image you can see while looking through a pair of binoculars, represented as a number of feet per thousand yards of distance. Note: A higher field of view usually means a less powerful magnification.

Image-Stabilized - Binoculars with a self-steadying feature, designed to counteract any hand-shaking of the user.

Individual-Focus - Unlike center-focus, individual-focus binoculars focus each eyepiece separately.

Light Transmission - The ratio of the total amount of light passing through the objective lens to the eye. Better levels of coating increase the amount of light reaching the eye.

Light-Gathering Power - The ability of the binoculars to collect light. This measurement is directly proportional to the size of the objective lens of the binoculars.

Magnification - The power of the binoculars. The magnification will tell you how many times bigger you will see an image, and is the first number in the set of numbers representing a pair of binoculars. Note: In most cases, the stronger your magnification, the smaller your field of view will be.

Near/Close Focus - The distance the user can be away from an object and still get a clear, focused view through the binoculars.

Nitrogen-Purged - The air inside the binocular tubes is replaced with nitrogen, which prevents mildew and mold inside the tubes. Nitrogen-Purged binoculars are commonly known as fog-proof. Note: In rare cases such as extreme humidity and elevation changes, some internal fogging may occur, though the fogging usually clears on its own after a few minutes.

Objective Lens - The large lens at the end of the binocular opposite the eyepiece. This lens gathers light into the eye.

Ocular Lens - The small lens in the eyepiece. In some cases (as in some roof-prism binoculars), this lens is the same size as the objective lens.

O-Ring Sealed - A special sealant on binoculars that makes them waterproof.

Phase Correction - A coating applied to the prisms of roof prism binoculars to prevent the light beam from splitting into two out of phase beams. This reduces image contrast and gives a clearer view.

Porro Prism - Binoculars with internal off-set prisms (as opposed to the aligned roof prisms) that bend the light rays inside the tubes to produce the image. These binoculars have eyepieces that are inset comparative to the objective lenses. Porro prisms have objective lenses spaced wider than roof prisms, and can produce a slightly better stereoscopic image.

Prismatic Binoculars - Binoculars that use internal prisms instead of a second lens to magnify an object. These binoculars aren't ideal for heavy-duty use, as the prisms can be broken or knocked out of alignment due to rough handling. However, the magnification strength of prismatic binoculars is much better than that of traditional field glasses.

Rangefinder Binoculars - Binoculars with a rangefinder built right in. A rangefinder is a tool used to calculate the exact distance between you and the object in focus.

Resolution - A binocular that has high resolution retains clarity and is able to distinguish fine detail.

Roof Prism - Binoculars with internal prisms that are aligned (as opposed to the off-set porro prisms) to provide a sleeker, more compact binocular. The image quality of roof-prism binoculars can suffer slightly because of the aligned prisms, although the top models of the roof-prism and porro-prism binoculars are now generally considered to have equal optical quality.

Wide-Angle Binoculars - Binoculars with a wider field of view (generally described as wider than 65 degrees).