Binoculars are the world's most used optical instrument (other than eyeglasses) and they have a lot of different uses. Choosing the right binocular for your application, though, can be confusing due to all the specifications and numbers. Here is a quick reference to what all those features and specifications mean. Consider when, where and how often you plan to use them in order to select a binocular with a combination of features that are right for you. (For more specific topics such on Astronomy Binoculars, Birding Binoculars, Marine Binoculars, Military Binoculars and Hunting binoculars, Opera Glasses and low power compact binoculars, Digital Binoculars, and Binocular Telescopes please see corresponding pages in our binoculars section.
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Binocular Magnification (Power)
Binoculars are often referred to by two numbers separated with an "x". For example; 8x32. The first number is the power or magnification of the binocular. With an 8x32 binocular, the object being viewed appears to be eight times closer than you would see it with the unaided eye. We do NOT recommend to use binoculars with magnification over 10x without a tripod - if you go with too much magnification in a hand-held binocular, your image will be very shaky! Also, many people ask - How far can you see with a binocular? - we always answer - You can see as far your eyes can see, but the objects will seem to be closer - magnified by the power of your binoculars. Please see this page for some sample digital pictures taken through giant Oberwerk binoculars with 25x and 40x magnification.
Objective Lens Size (Aperture)
The second number in the formula (8x32) is the diameter of the objective or front lens. The larger the objective lens, the more light that enters the binocular, and the brighter the image.
A binocular consists of two optical systems that are joined by a hinge and (typically) share a common focusing mechanism. The ability to create an image for both eyes simultaneously provides a realistic perception of depth. Binoculars are available in a great variety of sizes, magnifying powers and features to suit any purpose or preference.
||A standard or full-size binocular can be used for everything from nature observation to spectator sports.
||Compact Binoculars are smaller and lighter in weight and are a good choice to take along to the theater or concerts or on hikes and hunting trips.
|Wide angle Binoculars:
||Wide angle Binoculars are ideal for tracking fast-moving action across wide areas such as football fields, racetracks and wilderness terrain.
||A zoom binocular allows the user to increase the magnification in order to focus in on the details. From distant to near view, it's the best of both worlds.
||Waterproof Binoculars deliver clarity despite foul weather conditions including fog, rain and ice. O-ring sealed and nitrogen purged for reliable fogproof, waterproof performance.
Lens surface coatings reduce light loss and glare due to reflection for a brighter, higher-contrast image with less eyestrain.
||A single layer on at least one lens surface.
||A single layer on all air-to-glass surfaces.
||Multiple layers on at least one lens surface.
||Multiple layers on all air-to-glass surfaces.
A "fine focus" adjustment ring usually provided around one eyepiece to accommodate for vision differences between the right and left eyes.
Refers to the size of the circle of light visible at the eyepiece of a binocular. The larger the exit pupil, the brighter the image. To determine the size, divide the objective lens diameter by the power (an 8x32 model has an exit pupil of 4mm).
Exit pupil is a very rough guide to image brightness. Binoculars with large exit pupils provide brighter images under very low light conditions. For normal daylight viewing, an exit pupil of 2.5 or 3 is fine. For astronomy, an exit pupil of 5-7mm is preferred. An exit pupil larger than 7 is a waste of light since the human eye cannot open wide enough to accept an exit pupil larger than this. Exit pupil should not be taken too literally, since it treats all binoculars, regardless of lens coatings and optical quality as if they are the same.
Relative Brightness (RB)
Relative brightness, like exit pupil, is a rough guide to image brightness. It is simply exit pupil squared, so a binocular with an exit pupil of 5mm will have an RE of 25. As with exit pupil, 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.
This is a mathematical formula that predicts the mount of detail that can be seen in low light. Twilight factor is the square root of magnification times objective. A 10x40 will have a twilight factor of 20 (square root of 10x40). As with 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. No $50 binocular will ever equal a premium grade binocular for low light detail, even though they both have the same exit pupil.
EyeGlass Wearers - Eyecups
Binoculars come with Twist-Up, Pop-Up or soft rubber Fold-Down eyecups which go down for eyeglass wearers. These options allow everyone to see the entire field of view.
The distance a binocular can be held away from the eye and still present the full field of view. Extended or long eye relief reduces eyestrain and is ideal for eyeglass wearers.
Field of View (F.O.V.)
The side-to-side measurement of the circular viewing field or subject area. It is defined by the width in feet or meters of the area visible at 1000 yards or meters. A wide angle binocular features a wide field of view and is better for following action. Field of view is determined by two things. First is magnification. In general, as magnification goes up, field of view goes down. A 10x 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 thing that determines field of view in a binocular is the eyepiece design. Wide-angle design eyepieces of good optical quality, however, are expensive. Inexpensive binoculars with wide angle eyepieces are usually not as sharp as standard binoculars.
||A center focus binocular uses a single wheel to focus on objects. It can focus on objects both very close and far away, making it the most versatile and commonly used focusing system in a binocular.
|Individual Eyepiece Focus
||An individual eyepiece focus binocular requires you to focus each eyepiece when looking at an object, but once focused for your eyes, objects from 40 yards away to infinity are always in focus and require no additional focusing. This is a great system for medium range and long range objects, but it is not well suited for close in work. IF binoculars are most commonly found in marine binoculars and astronomy binoculars.
|No Focus or Focus Free
||This is an economy version of an individual eyepiece focus binocular, but the eyepieces are locked and set at the factory and cannot be adjusted. This means that you can never focus on objects closer than forty yards away and it also means that the binocular cannot be adjusted for differences in strength between your right eye and left eye. This is a serious shortcoming for most people, since most have one eye a bit stronger than the other.
Minimum Focus or Close Focus
The "minimum focus" or "close focus" is the nearest distance at which a binocular will focus on an object. A binocular will not focus on an object closer than this distance. This feature is important for some applications such as birding.
I.P.D. (Inter-Pupillary Distance)
The distance between the eyes differs from person to person. The adjustment for this difference is accomplished by opening or closing the hinged portion of the binocular to bring the eyepieces closer together or father apart.
Refers to the efficiency of an optic, or its ability to deliver the maximum amount of the light entering the objective lens to the eye.
Any time you have a binocular magnification over 10x or 12x, you should attach a binocular to a tripod to steady the image. Also, heavy binoculars with a last number of 70 mm or more usually need a tripod to support the weight. You can mount a binocular to a tripod if it is listed as tripod adaptable or if it is threaded for a tripod adapter. If it is listed as tripod adaptable, you will still need to purchase a tripod adapter, though a few large binoculars have this accessory built in.
Objective Lens Size
The second number in the formula (8x32) is the diameter of the objective or front lens on binoculars. The larger the objective lens, the more light that enters the binocular, and the brighter the image.
Near or Close Focus
The shortest distance at which the binocular is capable of providing a sharply focused image of an object. Especially of interest to bird watchers and nature buffs.
Most optical prisms are made from borosilicate (BK-7) glass or barium crown (BAK-4) glass. BAK-4 is the higher quality glass yielding brighter images and high edge sharpness.
The prism system of a binocular reduces the size needed to provide focal length and turns what would be an upside-down image right-side-up. There are two types of prism systems: roof and porro.
|Roof Prism Binocular System:
||In roof prism Binoculars the prisms overlap closely, allowing the objective lenses to line up directly with the eyepiece. The result is a slim, stream-lined shape in which the lenses and prisms are in a straight line.
|Porro Prism Binocular System:
||In porro prism Binoculars the objective or front lens is offset from the eyepiece. Porro prism Binoculars provide greater depth perception and generally offer a wider field of view.
Most people find that anything more than 35 ounces is too much to comfortably carry around the neck and a weight of less than 30 ounces is much better. If your current binocular is becoming a pain in the neck, you might want to look at a binocular harness, which supports the weight on your shoulders rather than your neck.
Resolution, or definition, is the ability of a binocular to distinguish fine detail and retain clarity.
Some Binoculars are O-ring sealed and nitrogen-purged for total waterproof and fogproof protection. These models can withstand complete immersion in water and stay dry inside. The interior optical surfaces won't fog due to rapid temperature change or humidity.
An armored binocular is a binocular with a housing covered by rubber or other synthetic material. Armoring does not make a binocular a waterproof binocular, but it does protect it from scratches, makes it more comfortable to hold and also "quiets" the binocular when it accidentally bangs against something.
A binocular with a variable power range, for example: 8-20x. Magnification is changed with a convenient lever or knob allowing you to increase the magnification when you want to get in close for detail from a distance.
Bushnell Binoculars Manual is available for download here
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