Binoculars come in a variety of sizes (defined by the objective lens size) for various outdoor pursuits. Here's a quick comparison.
Full-Size (common specs: 8 x 42, 10 x 50):
Mid-Size (common specs: 7 x 35, 10 x 32):
Compact (common specs: 8 x 25, 10 x 25):
Monoculars (single scopes):
Shop REI's selection of binoculars.
Binoculars are identified by 2 numbers. The first is magnification power, the second is the diameter of the front lenses, explained below.
Example: 7 x 35 binoculars have a magnification power of 7.
A magnification power of 7 means that an object will appear 7 times closer than it would to your unassisted eye. For example, if you view a deer that stands 200 yards away from you through 7x binoculars, it will appear as though it were 28.6 yards away (200 divided by 7).
So, the greater the magnification power the better the view, right? Not necessarily. Binoculars with magnification powers greater than 10 amplify the movements of your hands, making steady viewing difficult.
Objective Lens Diameter
The second number used in binocular identification refers to the diameter (in millimeters) of the objective lenses (those farther from your eyes; those closer to the "object" being viewed).
Example: 7 x 35 binoculars have objective lenses measuring 35mm.
The diameter of the objective lenses largely determines how much light your binoculars can gather. If you have 2 binoculars with exactly the same specifications except for objective lens diameter, those with the larger diameter objective lenses will capture more light. More light means a brighter view, particularly in low-light conditions.
Exit pupil is a number that indicates how bright objects will appear when viewed in low-light situations. A higher number means brighter images.
Point your binoculars at a light source, hold them about a foot in front of your face and peer into either eyepiece. See a small, bright dot? That circle of light is known as the exit pupil—the opening that permits light to exit each binocular barrel and reach the pupils of your eyes.
Exit pupil size (measured in millimeters) is calculated by dividing the diameter of the objective lenses by the magnification number.
Example: For 7 x 35 binoculars, 35 divided by 7 equals an exit pupil diameter of 5mm.
The wider the diameter of the exit pupil, the more light that can pass through, resulting in brighter, easier-to-see images when lighting is poor. If you anticipate regularly using binoculars in low-light situations—at dawn, dusk, within dense tree cover or while observing the night sky—seek out models with a high exit pupil number, preferably 4mm or higher.
For standard daylight viewing, exit pupil size is less important. In bright light human pupils narrow to roughly 2mm. All binoculars offer exit pupils that size or larger.
In dim light, however, our pupils can widen up to 7mm. A 7 x 50 binocular, for instance, offers an exit pupil size of 7.1mm—a good choice for low-light viewing. A large exit pupil also makes it easier to maintain a full image of an object if your hands move or shake.
Relative brightness is determined by squaring the exit pupil number. The higher the relative-brightness number, the brighter objects will appear to your eyes. This is useful in low-light situations.
Example: A binocular offers an exit pupil of 4.3. Square that number (4.3 x 4.3) to arrive at a relative brightness number of 18.5.
Do identical exit pupil size numbers produce identical brightness levels? Manufacturers of high-end binoculars say no, asserting that a variety of refinements—prism type, lens elements, component quality and optical coatings—all affect relative brightness.
Tip: If you wear glasses, look for eye relief of 11mm or more.
Field of View
This spec tells you the width of the area (usually in feet) that you can view at a glance, 1,000 yards from where you stand. A wide field of view is best to find and identify objects such as birds. Usually a higher magnification power results in a narrower field of view.
Once you've narrowed it down by category, your choice of a particular model will then depend on your budget and individual factors such as eyeglass compatibility. Remember, binoculars are only as good as the optics they use.
All illustrations in this article courtesy of Nikon.
By T.D. Wood
Read Author Bio
Last updated: 07/08/2013
In This Article
How are we doing? Give us feedback on this page.