Binoculars happen to be the world's most used optical instrument, except eyeglasses as my optician points out, yet they are also one of the most confusing to select or purchase. This is partially due to the many varied uses and choosing the right binocular for your application isn’t always easy because most of us have difficulty remembering what all of the specifications and numbers mean. This guide is intended to be a quick reference to what all those features and specifications mean. (For more specific topics such as Astronomy Binoculars, Bird Watching Binoculars, Marine or Military Binoculars and Hunting binoculars, as well as low power compact binoculars and digital binoculars, please contact us and we’ll see how we can help.

BINOCULAR NUMBERS

Labeling Binoculars
Typically binoculars are labeled with two numbers, for example 8x40, 7x35 and so on, which sometimes contain the abbreviation mm for millimeter at the end – we’ll explain this in just a few moments. Also, there are a few binoculars that use three numbers such as 10-20x50 - these are zoom binoculars. More about those later too, but let’s talk about the common numbers first.

What does the first number mean?
The first number - the 8 in the specification 8x40 or the 8 in the specification 8x25 - is the magnification power of the optics. Magnification simply means how many times the image is enlarged over normal when you look through the binocular. Another way to think of magnification is how many times closer the object appears to be in a binocular than it actually is. An 8x40mm binocular then, with 8x magnification, makes the subject appear to be eight times larger than it actually is, or to look at it the other way, eight times closer than it really is. An 8x50, 8x40, 8x20 are all 8x binoculars and possess an identical 8x magnification power.

In a zoom binocular, the first two numbers out of three represent the zoom range. For instance, in a 7-15x40, you can change the magnification from 7x up to 15x.

Choose this first number carefully - more is not always better and this is especially true of un-stabilized magnification. As magnification goes up, image brightness goes down, field of view (the amount of territory seen through the binocular) goes down and, just as importantly, image steadiness also goes down. In fact, image steadiness is the limiting factor on how much magnification can be used in a handheld binocular. This is because a binocular not only magnifies the image but also every shake and tremble in your hands or arms. In general most people find that a 10x binocular is the highest magnification that can be comfortably steadied without a support. Anything higher than a first number of 10x requires additional support, usually in the form of a tripod or rest.

What does the second number in a binocular mean?
The second number in a binocular - the 40 in 8x40 or 25 in an 8x25 - is the diameter (distance across) of each front lens, measured in millimeters. This number directly affects the performance of the binocular because when the magnification and quality are equal, the larger the second binocular number, the brighter and sharper the image. As a result an 8x40 binocular will produce a brighter and sharper image than an 8x25, even though both enlarge the image an identical eight times. The larger front lenses in the 8x40 also produce wider beams of light (exit pupil) that leave the eyepieces. The result of this difference makes it more comfortable for the human eye to view objects with an 8x40 than with an 8x25 binocular.

The down side of the larger front lenses in the 8x40 binocular also make the 8x40 bigger and heavier than the 8x25 binocular. An 8x25 may not be optically as good as an 8x40, but its smaller size makes it a better traveling companion. Remember, a compact binocular will almost certainly outperform any full sized binocular left at home or in the car because it was too heavy to carry.

By the way, the second number as most people mistakenly believe is not the field of view of the binocular, nor does it affect the field of view.

FIELD OF VIEW

What is the field of view?
The field of view is the amount of territory you see when you look through the binocular. Imagine a fence 1000yards away. If a binocular has a field of view of 372 feet at 1000 yards, you will see 372 feet of the fence. If the field of view is 250 feet at 1000 yards, you will see 250 feet of fence.

The field of view in binoculars is determined by two things. First is magnification. In general, as magnification goes up field of view goes down. A 10x binocular will show more detail in that fence at 1000 yards than an 8x, but it will not show you as wide a section of fence. The second item that determines the field of view in a binocular is the eyepiece design. Eyepieces designed with a wide viewing angle and of good optical quality, however, are expensive. Inexpensive binoculars with wide angle eyepieces are usually not as sharp as standard binoculars.

Note: The second binocular number IS NOT the field of view as most people commonly believe. An 8x25 binocular can have as wide a field of view as an 8x50 binocular. Lastly, a difference in field of view of 30 feet at 1000 yards in a binocular may sound like a lot, but out in the field it is not really significant.

OTHER BINOCULAR NUMBERS

The Exit Pupil
The exit pupil in a binocular is the actual width of the beam of light that leaves the eyepieces and is measured in millimeters. To see an exit pupil, hold the binocular at arms length and you will see a circle of light in each eyepiece; this circle is the exit pupil. To calculate the size of the exit pupil, divide the first binocular number into the second binocular number. A 10x50, 7x35, and 8x40 all have an exit pupil of 5.

The exit pupil number is a very rough guide to image brightness. Binoculars with large exit pupils provide brighter images under lower light conditions. For normal daylight viewing an exit pupil of 2.5 or 3 is usually sufficient for most applications, for astronomy an exit pupil of 5-7mm is preferred. An exit pupil larger than 7 provides no advantage since the human eye cannot open wide enough to accept an exit pupil larger than this. The exit pupil value should not be taken too literally since it treats all binoculars, regardless of lens coatings and optical quality as if they are the same.

A larger exit pupil does have other uses however. A large exit pupil is easier to keep centered on the eye which can provide an advantage when conditions make it difficult to hold the binocular steady, such as on the deck of a moving boat. That's why a 7x50 with an exit pupil of 7+ has always been the traditional marine binocular.

Relative Brightness
Relative brightness like exit pupil, is a rough guide to image brightness. The calculation of this number is simply the exit pupil squared. So a binocular with an exit pupil of 5mm will have an RE (Relative Brightness) of 25. As with the exit pupil number, relative brightness should not be taken too literally since it treats all binoculars, regardless of lens coatings and optical quality as if they are the same. In actual tests some premium grade compacts with a low relative brightness are actually brighter than some full size binoculars where the RE indicates the full size binocular should be brighter.

Twilight Factor
The twilight factor is a mathematical formula that predicts the amount of detail that can be seen in low light. To compute the twilight factor number, one calculates the square root of the magnification times the objective. Therefore a 10x40 binocular will have a twilight factor of 20 (square root of 10x40). As with the exit pupil and relative brightness, twilight factor should not be taken too literally since it treats all binoculars, regardless of lens coatings and optical quality as if they are the same. It should be noted that there are NO $50 binoculars that will ever equal the low light detail of a premium grade binocular, even though they both have the same exit pupil or relative brightness value.

RECOMMENDED APPLICATIONS

The following list presents the most popular sizes and other features for each listed use of binoculars:

Note: This article will be expanded over time to provide more information similar to that above on how to select a good pair of binoculars for your expedition or other use.