Binocular Basics

By Chuck Hawks

Binoculars are one of the handiest and most widespread of all optical instruments. Virtually anyone who spends much time outdoors owns (or should own) a pair of binoculars. They are almost a necessity for the astronomer, hunter, saltwater fisherman, boater, sports fan, and experienced traveler.

This article is intended to help the person who needs some basic information about binoculars. I hope to explain the specifications and terms commonly associated with binoculars. For those needing a more in-depth study of the subject, I recommend the excellent little book How To Choose Binoculars by Alan Hale.

Perhaps the first thing to understand is that binoculars are really just two small telescopes mechanically linked together. All of the facts and formulas that help you to understand telescopes also pertain to binoculars. (You will find telescope information on my Photography & Astronomy Page.) Each side of a pair of binoculars has a prime focal length, an objective lens, an ocular (eyepiece), an exit pupil, and so forth.

Binoculars are designed to give a correctly oriented, right side up view. This makes them ideal for terrestrial viewing, or for locating astronomical objects in the night sky. A good pair of binoculars is one of the handiest accessories for the astronomer, hunter, sportsman, traveler, birdwatcher, and nature lover. So whether you want to get a better view of the local football game, or find a big buck across a canyon, a good pair of binoculars is often indispensable.


Note that I wrote "good pair of binoculars" in the paragraph above. What I meant by "good" is quality, both optical and mechanical. Quality is the most important "feature" of binoculars. A product that is well made of high quality components is "good," and it is always worth the extra money over shoddy "popular priced" binoculars. Quality control costs the manufacturer (and you) money, but it is worth it because the result is a better performing pair of binoculars that can last a lifetime.

Many things affect both the quality and the price of a pair of binoculars. These include optical considerations like the type of glass selected for binocular lenses and prisms, the design of the eyepieces, the size and type of prisms, care in grinding and polishing the glass elements, and the type and coverage of anti-reflection coatings. Important mechanical considerations include the material the lens barrels themselves are made of, the construction of the lens barrels (one or two piece), the way the lenses and prisms are mounted and retained in place (by sticky tape, glue, or threaded retaining rings), the focusing mechanism, and the outside finish of the binoculars.

In many cases the brand name is a guide to quality. Companies like Celestron, Leica, Minolta, Nikon, Pentax, Steiner, Swarovski, and Zeiss have spent decades earning a reputation for high quality optical products, and they are unlikely to produce a clearly inferior product. Other companies, like Tasco, Jason, and Bushnell have built a reputation on low price. You usually get what you pay for.

The dealer you buy from matters, as well. A good binocular dealer can help you select the binoculars that best fit your needs. Specialty astronomy shops, camera stores, and sporting goods stores (the traditional kind that sell guns, fishing tackle, and binoculars, not the kind that sell apparel and shoes) are usually the best sources for both good quality binoculars and good information.

Many sales clerks know virtually nothing about binoculars, sometimes not even how to focus them correctly. Do not buy from a clerk or a store that cannot provide the information you need to make an informed buying decision. If the sales person does not know at least as much as you do after reading this article, shop elsewhere.

Buy where you get the help and information you need. Good service usually costs a little more. Experienced and knowledgeable employees deserve higher salaries, and locally owned specialty stores probably have higher overhead costs than department stores and chain outlets. But it is well worth a few extra dollars to deal with people who will make the effort and take the time to see that you get the right binoculars. Good binoculars are a lifetime investment.


Prisms are what let you see a correctly oriented image when you look through a pair of binoculars. There are two types of prisms in common use, Porro prisms and roof prisms.

Roof prisms are essentially in line inside the optical tubes, and make for a more compact set of binoculars. Roof prism binoculars have straight tubes (the front/objective lens is in line with the rear/ocular lens), and are therefore more compact, an important consideration for the sportsman. They usually have two pivot points between the tubes, and are more difficult to adjust to the spacing of your eyes. Roof prisms can give an optical image equal to the best Porro prisms, but for technical reasons they usually do not. To be really good, roof prism binoculars have to be in the high price class. Do not attempt to economize on roof prism binoculars.

Porro prism binoculars can be identified by their offset tubes; the objective lens is not in line with the ocular lens. The front lenses are usually closer together than the rear lenses, but the reverse can also be true, particularly in compact models. The Porro prism design is usually optically superior to the roof prism design, especially in medium priced class binoculars. Porro prism binoculars have a single pivot between the two halves of the binocular, and are therefore easy to adjust for the distance between your eyes.

Like roof prisms, not all Porro prisms are created equal. BAK-4 prisms are the best; they are made of superior optical glass that produces clearer images. These are what you want in your binoculars. BK-7 prisms are also used, usually in lower priced binoculars. These are satisfactory, but they are inferior to the BAK-4 prisms. Some manufacturers will not tell you what kind of prisms they use, usually because they are of inferior quality.

Lens coatings

Most binoculars have antireflection coatings on their air to glass surfaces. These coatings assist light transmission. They are what produce the blue, red, or green reflections you see when you look into the front (objective) lens of a pair of binoculars.

But note how the manufacturer describes his coatings. "Coated" means a single layer antireflection coating on some lens elements, usually the first and last elements--the only ones you can see. "Fully Coated" means that all air to glass surfaces are coated. This is good. "Multi-Coated" means that at least some surfaces (again, usually the first and the last) have multiple layers of antireflection coatings. (The others presumably have single layer coatings.) Multiple layers are about an order of magnitude more effective than a single layer. "Fully Multi-Coated" means that all air to glass surfaces have received multiple layers of antireflection coatings, and this is what you want in your binoculars.

The latest fad in coatings is ruby or red multi-coatings. These are intended to reduce glare in bright light.


Collimation refers to the optical and mechanical alignment of the binoculars. If a pair of binoculars is out of collimation, after prolonged use it may feel as if they are trying to suck your eyes out of your head.

Cheap binoculars are often (perhaps usually) shipped from the factory out of collimation. Good binoculars are carefully collimated, often with laser instruments. This requires time and expense at the manufacturing level, and raises the price at the retail level.

Magnification (power)

Binoculars are commonly described by using a pair of numbers, as in "7x50" or "8x25." The first of these numbers refers to the magnification offered by the binocular. Magnification is why most people buy a pair of binoculars. In the examples above, "7x" means the binocular makes whatever you look at appear seven times closer than it does to the unaided human eye. "8x" means the binocular makes whatever you look at eight times closer than the unaided human eye. "10x" makes things look ten times closer, and so on. The first number used to describe binoculars always refers to their magnification. Common binocular magnifications are 6x, 7x, 8x, 9x, and 10x.

There are also variable power (zoom) binoculars, such as 7-21x50. These almost always perform much better at the low power setting than they do at the higher settings. This is natural, since the front objective cannot enlarge to let in more light as the power is increased, so the view gets dimmer. At 7x, the 50mm front objective provides a 7.1mm exit pupil, but at 21x, the same front objective provides only a 2.38mm exit pupil. Also, the optical quality of a zoom binocular at any given power is inferior to that of a fixed power binocular of that power. In general, zoom binoculars are not the bargain they seem to be.

Remember that everything (including movement) is magnified when you look through a pair of binoculars, especially your own shakes and tremors. So the higher the power, the harder it seems to hold the binoculars steady. 6, 7, or 8 power binoculars are easier for most people, even those with very steady hands, to hold reasonably still. The higher powers sound like a good deal, but often result in jiggly, blurred views. This is why 7x binoculars are chosen by so many experts, including the military.

Power affects brightness. Other things being equal, the higher the power, the dimmer the view. And power also affects the field of view of the binoculars. Again, everything being equal, the higher the power, the smaller the field of view. So, as you can see, power must be balanced against other desirable characteristics when choosing binoculars.

Objective lens (diameter)

The second number most commonly associated with binoculars refers to the diameter of the objective lens in millimeters. Thus in "7x50," the "50" means that the front lenses of the binoculars are 50mm in diameter, which is large for hand held binoculars.

This is very important information, because the larger the diameter of the front objectives, the more light can enter the binoculars and be focused to your eyes. Thus, other things being equal, you can see better in dim light with binoculars that have large front lenses. For example, 7x50 binoculars are often called "night glasses" because they seem so bright in dim light. Read more about this under "Exit pupil." Of course, a large front objective makes for larger, and heavier, binoculars. Which is why compact binoculars always have relatively small front lenses.

Exit pupil

The magnification and the diameter of the objective lens determine the size of the exit pupil. The diameter of the exit pupil determines how much light is transmitted to your eye. The exit pupil can be seen by holding the binoculars at arm's length and looking through the eyepieces. The pencil of light you see is the exit pupil.

The actual diameter of the exit pupil is easily computed. Divide the diameter of the front objective lens (in millimeters) by the magnification of the binocular. For instance, take a pair of standard size 7x50 binoculars. Divide 50 (the diameter of the objective) by 7 (the magnification) and you get approximately 7.1 (50/7=7.1). 7.1mm is the diameter of the exit pupil for 7x50 binoculars. Now let's figure the exit pupil of a pair of compact 8x25 binoculars. Divide 25 by 8 and you get 3.1 (25/8=3.1). So the exit pupil of 8x25 compacts is only 3.1mm. A lot less light reaches your eye from compact binoculars than it does from standard size binoculars. Light is what you are sacrificing to get compact size and weight.

Why does the diameter of the exit pupil matter? It doesn't as long as there is enough ambient light so that the pupils of your eyes are smaller than the exit pupils of your binoculars. But when the ambient light gets dim, and the pupils of your eyes adapt by enlarging, the exit pupils of your binoculars may become the limiting factor. With the 8x25 compacts in the example above, when it gets dim enough for the pupils of your eyes to exceed 3.1mm in diameter, the binoculars are restricting the light available to your eyes. Ideally, human eyes in excellent condition can achieve about a 7mm pupil opening, so a 3.1mm exit pupil from your binoculars can be quite limiting in dim light. You can probably see more without your binoculars. But the 7x50 binoculars in the first example above have 7.1mm exit pupils, as large as young, fully dark-adapted human eyes, so they never limit what you can see, even at night.

The human eye loses its ability to adapt to dim light as it ages, so a middle-aged person's maximum pupil size is typically down to around 5mm. Elderly eyes are often limited to about a 4mm pupil maximum pupil. So as we age, the exit pupil size we need decreases.

Field of View

The field of view is the area seen through your binoculars. It is properly measured in degrees. The larger the field of view the more area you can see. Field of view is particularly important when observing moving subjects, like animals or birds, or at sporting events.

Relative brightness index (RBI)

RBI endeavors to measure image brightness. It is computed by squaring the exit pupil. For example, 7x35 binoculars have a 5mm exit pupil (35/7=5). So their RBI is 25 (5x5=25).

A RBI of 25 or greater is considered good for use in dim light. Since you already have learned (above) how to compute the actual exit pupil size, and what it means, RBI is largely redundant.

Twilight factor

This is a somewhat subjective measurement that purports to reveal how much detail you can see in twilight conditions (however that is defined). It tends to favor magnification, which is good for binocular sales.

For instance, Celestron computes the twilight factor of 7x50 binoculars as 18.7, and the twilight factor of 10x50 binoculars as 22.4, even though the former has a 7.1mm exit pupil, and the latter only a 5.0mm exit pupil. The increased magnification presumably makes up for the decrease in brightness in "twilight conditions" (when the eye is not yet fully dark-adapted). This rather artificial measurement can be useful to the hunter and birdwatcher, since animals are often spotted just before sunrise, and just after sunset.

How to focus binoculars

It is surprising how many people do not know how to focus binoculars correctly. There are two common focusing systems used in binoculars.

The first is individual eyepiece focus. This system is simple to understand, and easy to manufacture. It also lends itself well to sealed optical tubes, and thus is usually the focusing system used for waterproof binoculars. Individual eyepiece focus means that to focus the binoculars to your eyes, you simply focus the left eyepiece to your left eye and the right eyepiece to your right eye. There is no centrally located focusing mechanism. It is done like this. Look at something in the distance. Close the right eye (or cover the front of the right binocular), and focus the left eyepiece to your left eye. Close the left eye (or cover the front of the left binocular), and focus the right eyepiece to your right eye. You are finished, until you need to look at something at a different distance, in which case you need to repeat the process.

Because individual eyepiece focus is time-consuming, center focus is more common. Unfortunately, very few people understand how to correctly use center focus binoculars. Here is how it is done. Aim your binoculars at something in the distance. Close the right eye (or cover the front of the right tube), and focus the left side of the binocular to your left eye using the center focus control, which is concentric with the pivot shaft between the binoculars. (Note: the left eyepiece itself does not focus on center focus binoculars.) Next, close your left eye (or cover the front of the left tube), and focus the right eyepiece to your right eye. DO NOT touch the center focus control while you are focusing the right eyepiece to your right eye. Now you are finished. What you have just done is adjust the binoculars for your individual eyes. (Practically everybody's left and right eyes are different.) From now on, you only need to adjust the center focus control when you look at things at different distances. Center focus is faster and easier to use than individual eyepiece focus, once you have initially set the binoculars for your eyes.

Binoculars for Travel

Travel binoculars need to be light and compact so that they will fit easily into carry-on luggage, or even a pocket. This is where the compact roof prism type of binocular really comes into its own. Great light grasp can be sacrificed for portability, since travel binoculars will ordinarily be used during daylight hours. Magnification must permit handheld viewing as aids to steady support will be few and far between. Something between 7 and 10 power would be a reasonable choice. Very high optical quality helps make up for limited light gathering ability.

My personal travel binoculars are compact 9x Leupold roof prism glasses. I would not have them any more, nor much less, powerful. 8x might be even better. They came with a soft leather pouch case that has a convenient belt loop, and that is how I most often carry them, although they will also fit into a shirt or jacket pocket.

Binoculars for astronomy

A good pair of binoculars are very handy for locating objects in the night sky. Once an object has been located with binoculars, it is easy to train a telescope on it for a more detailed view. The binocular astronomer needs very high quality, very bright binoculars. For general hand held use 7x50, 8x56, and 9x63 binoculars will serve very well. Pick the highest power that you can hold steady.

Giant binoculars are in a class by themselves for binocular astronomy. These require a solid tripod mount, but reveal spectacular views of large objects like open star clusters. The 20x80 size is the most popular, and perhaps represents the best compromise between magnification, brightness, and field of view for general astronomical observing.

Binoculars for hunting and fishing

For finding things in the field, you need fairly bright binoculars. The navies of the world generally use 7x50 binoculars on their ships. These offer a 7.1mm exit pupil and gather all the light young eyes can use. They also offer a good field of view and as much magnification as is practical from a moving vessel. However, they are comparatively heavy and bulky. This is not a problem for a fisherman operating from a boat and 7x50 marine binoculars are the first choice for the salt water fisherman. However, the hunter needs something more compact.

For field applications a 4mm to 5mm exit pupil is usually satisfactory and 6x30, 7x35, 8x30, or 9x35 binoculars are probably the most useful compromise for hunting. They are bright enough to allow the observer to see into shadowed areas, or in dim light, and compact enough to not be an excessive burden to carry. Higher power binoculars are hard to hold steady without external support and objective lenses of 40mm or larger tend to make for heavy and bulky binoculars that are a burden to carry.

The woods hunter will be well served by a glass with a generous field of view, like a 6x30. The mountain hunter will probably favor higher power, since he will use them to spot game at greater distances. 8x30 or 9x35 binoculars will do well. A pair of the common 7x35 size binoculars is probably about as good for all-around field use as any. Whatever magnification best fits your needs, be sure to buy top quality binoculars and you will not be disappointed.

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Copyright 2001, 2016 by Chuck Hawks. All rights reserved.