Binocular Terms


Binocular – Before you can determine what type of binocular you really need, you should understand the following binocular terms.

Exit Pupil:  The exit pupil is the small circle of light that appears in the eyepiece.  The formula for determining the exit pupil size is the objective lens diameter size in millimeters divided by the magnification.  The larger the exit pupil on the binocular, the less critical the position of your head is in relation to the binocular.  Also keep in mind, that an older person’s eyes may only dilate to about four millimeters in low light situations while the younger person’s eyes may open up to seven millimeters or more.  Therefore, a larger exit pupil size on a binocular will become less of a deciding factor for the older eye.

Eye Relief:  The eye relief is the distance that your eye must be from the eyepiece on a binocular to get a full, clear picture.  Eye cups provide the eyeglass wearer easy head positioning for the appropriate eye relief.

Field of View (FOV):  The FOV is measured in feet at 100 yards. This is the amount of view you see through a binocular from right to left at that distance.  FOV is inversely effected by magnification – as magnification increases, the smaller the FOV, and conversely, as the magnification decreases, the greater the FOV.  For example, lower power provides a better configuration if you will need to be acquiring targets at a close range with a binocular.

Focus Systems:  There are three focus systems for a binocular:

  • Center focus is the old standard method of focusing using a finger wheel mounted between the two sides of the binocular.
  • Individual focus means that each binocular eyepiece can be focused independently.
  • Focus free types use an optical trick to permanently focused the binocular to all distances greater than a minimum distance.  These are really useful for viewing fast moving sports action because you avoid the need to keep re-focusing as the action moves at varying distances.

Lens Coating: See anti-reflective coating.

Magnification:  The magnification is actually a relationship of two independent optical systems– the objective lens and the ocular (eyepiece) on a binocular.  It is calculated by dividing the focal length of the objective lens by the focal length of the eyepiece. Magnification, in simple terms, is how many times larger an object will appear than with your naked eye.  A binocular comes in variable or adjustable powers that offers a range of magnifications.  This information is identified by a range of numbers that are separated by a hyphen on the binocular.  Beware of the notion that “the more magnification – the better.”  In reality, the more magnification you have, the less light that gets to your eyepiece thus reducing the image brightness, contrast, and clarity, while also reducing the binoculars’ field of view.

Objective Lens:  The binocular objective lens diameter size is measured in millimeters (mm).  The larger the objective lens, the brighter the image and the higher the contrast and clarity.  However, the negative side to a larger objective lens is that the bigger glass elements are heavier in weight.

Twilight Factor:  The twilight factor is calculated by taking the square root of the product of the magnification and the aperture.  The higher the twilight factor, the better the resolution of the binocular when observing under dim light conditions.  This partially explains why some spotting scopes of identical exit pupil size will have differences in observed image sharpness and detail in twilight.

It is also helpful to be able to decode the words and letters used by manufacturers to describe their binocular models designations.  For example, “10 x 40 B/GA” means the is 10 power, 40mm objective lens, long eye relief, and an armored binocular.  Here’s what the code letters mean:

A or GA:  rubber armored.
B:  either a porro prism with a Bausch & Lomb-style one-piece body (American or Japanese usage), or a binocular with long eye relief for eyeglass use (German usage, from the German word “briller,” meaning “eyeglass”).
C:  compact roof prism.
CF:  center focus (not “close focus” as many people think).
D:  roof prism (abbreviation of “dach,” the German word for “roof”).
H:  H-body roof prism.
IF:  individual focusing eyepieces.
P:  phase-corrected prism coatings.
W: WA, or WW:  wide angle.
Z:  porro prism with Zeiss-style two-piece Z-body.


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