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A Quick Guide* to the Numbers on Binoculars

Before you buy a pair of binoculars for outdoors, hunting, or birding, it’s important to understand what the numbers on those binoculars mean. These numbers tell you a lot about the product. And when you understand these optics and the differences between them, you’ll be in a better position to buy the binoculars that offer the best viewing experience you're looking for.  

Magnification is usually considered most important, but field of view, brightness, ease of handling (weight, feel, ergonomics), suitability for eyeglass wearers, and overall construction should also be taken into account. So let’s dig into it!

*The information in this article was taken from the Nikon Binoculars Handbook



Binocular Optics 101—What do the basic numbers mean?


Magnification

Magnification, expressed by a numerical value, is the relationship between a subject’s actual proportions and its magnified size.

This is the first number you see on binoculars.

With 7x magnification, for example, a subject 700 yards away appears as it would when viewed from 100 yards with the naked eye.

The higher the number, the closer your subject will appear through the lenses.

As a rule, magnifications of 6x to 10x are recommended for handheld outdoor use.

With magnification of 12x or greater, shaking by hand movement can create an unstable image and less enjoyable viewing.


Objective Lens Diameter

The number following magnification is the objective lens diameter—or aperture.

For example, in the the Nikon MONARCH M7 8x42, the second number tells us that the objective lens size is 42mm.

Combined with the quality of lens and prism coatings, this number determines the amount of light gathered to form an image.

If you are regularly observing in poor light conditions, such as early dawn or dusk, or in forested areas, you may need a larger objective lens.

But large-diameter objective lenses make binoculars heavier, so 50mm is the general limit for handheld use.

And if you are trying to pack light for long viewing expeditions, this may affect your decision on which binoculars to buy.


Field of View

The real angle of view is the third number printed on all Nikon binoculars.

It represents the segment of a 360° circle that the binocular is designed to view. The higher this number, the more of the subject you see from side to side.

Pretty much anything over 6° is considered to be a good angle of view.

This number is also used to determine if the binoculars can be rated as a wide-angle or wide field-of-view instrument.


What do the advanced numbers and specifications on binoculars tell you?


Eye Relief/Eye Cup Usage

Eye relief is the distance from the outer surface of the eyepiece lens to the position where the exit pupil is formed (eyepoint).

Looking through binoculars from the eyepoint, you can obtain the whole field of view without vignetting.

Eye relief lets you know how far away from the eyepiece your eyes can be while still being able to enjoy the full field of view that the binoculars offer.

For eyeglass wearers, it's recommended to use binoculars with a longer eye relief (high eyepoint).


Exit Pupil

The exit pupil is the bright circle that can be seen in the center of each eyepiece when you hold the binoculars about 12 inches away from your eyes with the objective lenses pointed toward a bright light. The larger the diameter is, the brighter the viewfield is, which is an important consideration when using binoculars in dark situations and for astronomical observation.

Exit pupil = The effective diameter of the objective lens ÷ Magnification

    • With 8x42 binoculars, the formula is 42 ÷ 8 = 5.3. Therefore, the diameter of the exit pupil is 5.3mm. The adjacent image indicates the brightness of the image in view.

The relationship between the dilation and contraction of your eyes (pupil size) and the size of the Exit Pupil determines light delivery potentials.

    • Smaller exit pupils affect brightness enormously, because the pupil of the human eye controls the amount of light entering our eye, by shrinking when light is brighter, or growing when light dims.

The human eye pupil diameter ranges from about 2mm in bright light, to a maximum of about 7mm in total darkness.

    • For our optics-aided eyeball to take advantage of available light, the exit pupil in a binocular should be at least the size of our own eye’s pupil in any given situation. If smaller, your eye will be “light starved” (not enough light reaches your eye). This normally happens at dawn and dusk.

Of course, during daylight hours, or in a well-illuminated venue (stadium, arena, or concert hall), your pupil can contract to be as small as 2mm or 3mm. In these usages, a 3mm exit pupil is sufficient and allows the use of a smaller, more portable binocular.


Real Field of View

Real field of view is the angle of the visible field, seen without moving the binoculars, measured from the central point of the objective lens.

The larger the value is, the wider the viewfield available.

For example, binoculars with a wider field of view are advantageous for locating fast-moving wild birds within the viewfield.

This also applies for finding small nebulas or a cluster of stars in astronomical observations.


Apparent Field of View

Apparent field of view is the angle of the magnified field when you look through binoculars.

The larger the apparent field of view is, the wider the field of view you can see even at high magnifications.

With the conventional method used previously, the apparent field of view was calculated by multiplying the real field of view by the binocular magnification. (With this formula, apparent field of view wider than 65° is called wide field of view.)

After revision, Nikon’s figures are now based on the ISO 14132-1:2002 standard.


Field of View at 1,000 Meters

Field of view at 1,000 meters is the width of the visible area at a distance of 1,000 meters, which can be seen without moving the binoculars.

For example, with 8x42 7.0° binoculars:

Field of view at 1,000m = 2 x 1000m tan (7.0÷2) = 122m


Field Flattening

Nikon’s field-flattener lens system technology minimizes curvature of field.

These are irregularities that occur when focusing on the center of the field of view causing the periphery to go out of focus and vice versa.

The result of field flattening is a sharper, clearer images all the way to the lens periphery!


Quick Tips for Buying Binoculars

Now that you know more about what the numbers on binoculars mean, you can take away some tips for buying your own.

  • Magnifications of 6x to 10x are best for handheld outdoor use.
  • In low light conditions, a greater objective lens size is better—but remember this makes your binoculars a bit heavier.
  • Greater magnification means a smaller field of view.
  • Look for a long eye relief (more than 15mm) if you wear glasses.
  • An exit pupil of more than 4mm is recommended for night viewing.

Nikon offers an extensive lineup of binoculars, including several of the world’s most popular series for a diverse range of applications.

Each model features various technical specifications that can help you in making the right selection.