The diameter of the binocular lens, measured in millimeters.

Aspherical Lenses

If all other binocular features are equal, aspherical lenses provide the binocular with brighter, sharper images.

BaK-4 Glass

Binoculars using this fine, high density barium crown glass, eliminate internal light scattering so the images seen through these binoculars are sharper.

Center Focus Binoculars

Binoculars using a center focus mechanism allow both eyepieces to be adjusted simultaneously for rapid focus.

Chromatic Aberration

This is a defect of optical lenses used in binoculars. Different wavelengths (producing different colors) are diffracted, or bent, at different angles and produce colored halos around images.

Close Focus

The close focus (near focus) of a binocular will vary between binoculars. If in viewing things like butterflies you need to be close to the subject, choose a binocular with a low close focus number.

Coatings

A thin layer of an anti-reflection coating is applied to the glass surface in a binocular. This coating reduces the amount of light lost as the light passes through the glass surface. The result is that more light will actually passes through your binocular and reaches your eye. Better coatings with more complete and complex coating schemes, improve the brightness and overall optical performance of the binocular.

Collimation

Collimation refers to the proper alignment of lenses in the optical system of the binocular.

Compact Binoculars

Binoculars so small they fit in a pocket or purse are called compact binoculars. The small, compact nature of these binoculars makes them ideal for day use. Available as roof prism binoculars or reverse-Porro prism binoculars.

Depth of Field

This refers to what you actually see in focus at any one time. It is the area in focus between the foreground and background of the viewing area.

Diopter Adjuster

All better quality binoculars will allow for separate adjustment of the focus for one eyepiece (usually the right). This is done to compensate for differences between eyes.

Exit Pupil

The exit pupil of a binocular is the point at which all of the light rays that entered the objective lens and passed through the binocular exit through eyepiece to form a magnified, circular image. If you hold the binocular away from your eyes and look through the ocular lens, you will be able to see the clear circular exit pupil.

To Calculate the Exit Pupil of a Binocular
Divide the measurement of the binocular's objective lens by the magnification of the binocular. For example, with an 8x40 binocular you would divide the 40mm objective lens by the magnifcation of 8 power for an exit pupil of 5mm.

Eye Relief

Eye relief in a binocular refers to the distance images are projected from the ocular lens to their focal point. The eye relief of a binocular can vary from 5mm to as much as 23mm.
Note: Without proper eye relief, the eyeglass wearer will not see the full field of view.

Field of View

The field of view is the widest dimension of the circular viewing area seen through a binocular. This measurement may be listed on the binocular in either degrees or feet, measured at 1000 yards.
Note: One degree equals 52.5 feet/1000 yards.

Focal Point

This is a point where the light rays from an image come sharply into view after passing throught the binocular.

Full Size Binoculars

Full size binoculars offer better light gathering ability than compact binoculars because of a relatively large objective lens. For example, an 8x42 binocular is a full-sized binocular (an 8x25 binocular is called a compact binocular).

Fully Multi-Coated Optics

Binoculars that have fully multi-coated optics have multiple coatings on all air-to-glass surfaces.

Individual Focus Binoculars

Each eyepiece of the binocular focuses independently of the other (unlike with a center focus binocular which adjusts both eyepieces at the same time).

Light-Gathering Power

The light-gathering power of a binocular is determined by the surface area of its objective lens.

Light Transmission

Light transmission describes the percentage of available light passing through a binocular to the viewer's eye. Better coatings on the optics of a binocular increase the amount of light that reaches the eye.

Magnification

Higher magnification allows you to see more detail through the binocular.
Note: As the magnification of a binocular increases, the field of view decreases.

Multi-Coated Optics

Binoculars with multi-coated optics have one or more surfaces coated.
Note: Within a binocular the coatings may vary from one surface to another. For example, one surface may not have any coatings, another may have only one coating, while another may have several coatings.

Nitrogen Purged

The atmospheric air within the binocular is replaced with nitrogen. This prevents mold, mildew or acid etching of binocular lenses and prisms that atmospheric air (containing moisture) would permit. Most waterproof binoculars are nitrogen purged.
Note: Temperature changes, elevation changes, or extreme humidity can create internal fogging. Most of the time this moisture dissipates on its own in a matter of minutes.

Objective Lens

This lens is at the end of the binocular away from the eyepiece. It gathers light to the eye.

Ocular Lens

This is the lens in the eyepiece. It is usually smaller than the objective lens except in the case of some roof prism binoculars.

Ruby Coatings

The objective lenses of a binocular with ruby coatings will be a bright reddish-orange. Since red light is reflected the colors seen through binoculars with ruby coatings are skewed to the cool end of the spectrum.
Note: Another result of using ruby coatings is a shortened color spectrum which may increase the contrast and resolution of a binocular.

Phase Correction

In a roof prism binocular the light waves are reflected off the surfaces and split into two out-of-phase beams of light. Application of a phase correcting coating forces the light back into phase.
Note: Phase correction in a binocular greatly enhances color fidelity, also affecting brightness, contrast and resolution.

Porro Prism Binoculars

The Porro prism binocular has the "traditional" binocular appearance with offset prisms for image-erecting.

Roof Prism Binocular