The size in inches or mm of an objective lens or mirror used in a telescope or binocular. A 25x100mm astronomical binocular will have an aperture of 100mm or almost 4 inches in diameter.

The distance in which objects appear when looking through binoculars. To calculate the apparant distance, divide the actual distance by the power. For example, using an 8x40 binocular viewing an object 100 feet away, your apparant distance is 12.5 feet. (100 feet / 8x = 12.5 feet)

A non-aspherical lens element that helps to reduce chromatic and other lens aberrations.

The designation assigned to the type of glass used for making the highest quality prisms.

A practical and easy method of focusing, this feature allows you to adjust the focus of both barrels simultaneously.

DCF is also a Central Focusing format and involves the use of a roof prism design.

DIF have individual eyepiece focusing formats which involve the use of a roof prism design.. Many waterproof models use this design although more expensive waterproof models have the central focusing wheel for convenience.

The ability to hold or "lock in" the diopter setting on the right-side eyepiece so your binoculars remain in the exact setting for your vision requirements. You adjust the left side focus of the binocular with the center focus wheel. Once this is done you then use the diopter setting on the right eyepiece to focus in the right eye. Once completed the binoculars will automatically focus properly for both eyes when object either closer or further from the observer are brought into focus using the center focus wheel.

The exit pupil determines the size of the beam of light that leaves the eyepiece of the binoculars. The larger the exit pupil, the lighter the image under low-light conditions but be cautious as too large an exit pupil (in some cheaper models) may cause difficulty for certain users to align their eye properly with the light beam and the field of view will appear to wink in and out as the eye is moved. This can also happen with long eye relief eyepieces. Another note is that exceptionally large exit pupils may not provide brighter images for older people since their pupils no longer open as wide as they did in their younger years. To calculate the exit pupil, divide the objective lens size by the magnification power. For example, using an 8x40 binocular, your exit pupil is 5. (40mm / 8x = 5 (exit pupil size))

Eye relief is the maximum distance you can place your eyes from the binoculars and still observe the full field comfortably. Longer eye relief is especially helpful for eyeglass wearers. It should be noted that some long eye relief binoculars may have such a long relief that some users may find the glasses difficult to use since the exit pupil may be too large close to the eyepiece.

The angular field of view is indicated on the binoculars in degrees or in width of feet (meters) as the area you can see at 1000 yards (Meters). The larger the angle or number of degrees, the wider the area you will be able to see. This is especially of concern when buying zoom binoculars. Make sure that the field of view at the highest power will be large enough to be useful at the likely distance of the object to be viewed.

Focusing moves the lenses inside the binocular so that magnified beams of light are focused on your eyes

Optical quality glass that has been carefully ground to magnify objects without distorting them.

Magnification usually preceeds the size of the objective lens. For example a 10x50 binocular would indicate a magnification of 10x with the main objective lens being 50mm. Zoom binoculars may be shown as 10-30x60mm which would translate into zoom from 10x to 30x magnifications with the main objective lens being 60mm in diameter.

The largest lens mounted in the front of the binocular is known as the objective lens and is usually measured in millimeters of diameter. This diameter determines the binocular's light gathering ability as well as its resolution (ability to show detail). A larger objective lens diameter will collect more light allowing for greater detail and image clarity. Beware of very high power models with small objective lens sizes. Usually a ratio of 10x per 25mm of diamter is the highest recommended usable power in binoculars. There are exceptions but remember that as the power increases the image brightness decreases as does the field of view. The other concern is that errors in optical alignment which are insignificant at 10x, at 30x become a considerable inconvenience.

The eyepiece lens in an optical instrument. It focuses the image to make it visible to your eye.

PIF stands for Individual eyepiece focusing and involves the use of a porro prism design.. Many waterproof models use this design although more expensive waterproof models have the central focusing wheel for convenience.

A combination of right-angle prisms that ensures the image is the right way up for viewing. The use of prisms also shortens the distance between the objective and eyepiece lenses, reducing the size and weight of the binoculars.

There are two types of prisms which are widely used in binoculars. The Roof prism which is lighter in weight and somewhat more expensive than the Porro prism which are used to invert the image. You may notice that many dealers advertise BAK4 prisms and not too many telling customers about BAK7. Most higher priced binoculars have BAK4 prisms which refer to the glass type used in their manufacture. BAK4 is a slightly higher quality glass than BAK7 having fewer impurities as well as different refractive indexes which translates into better image quality near the edge of the field. This being said, in wide low power fields the "usable" field of view produced by both prisms are virtually identical. In fact, under most conditions the user will be unaware of the difference.

A system in which prisms are positions in an in-line assembly, resulting in a straight-barreled, more compact binocular.

Nearly all manufacturers put some type of coating on at least one of the optical surfaces in their binoculars. But Pentax puts a seven-layer coating on all optical surfaces. This means more available light is transmitted to your eye. Glare is reduced and image contrast is enhanced.

UCF stands for Central Focusing and involves the use of a porro prism design.

Will survive submersion up to one metre underwater for 5 minutes. Not suitable for use underwater.

Can be used in rain or snow and rinsed in running water to remove mud and dirt.

Binoculars with an apparent field of view greater than 65º offering a broader scope of view.