When shopping for binoculars, you’ll discover wide price ranges on similar-looking styles. Understanding binocular specs, such as magnification, objective lens diameter and exit pupil will help you narrow down which pair works best for your needs.
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)
Best for serious wildlife viewing and for use on boats. Full-size binoculars capture more light and perform better in low-light situations. They usually provide steadier images and a wider field of view, so they're great for bird watching, but they're too big and heavy for backpacking.
Mid-Size (common specs: 7 x 35, 10 x 32)
Best all-around choice for wildlife and sports use. While a bit heavy for backpacking, these binoculars balance moderate size and above-average light transmission.
Compact (common specs: 8 x 25, 10 x 25)
Best for daytime outdoor activities. These are the lightest, smallest binoculars for backpacking, but they’re less comfortable during extended periods of use.
Binocular Magnification Power
Binoculars are identified by 2 numbers. The numbers on binoculars show magnification power and lens diameter.
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).
Be aware that binoculars with magnification powers greater than 10 amplify the movements of your hands, making steady viewing difficult.
Binocular 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.
Binocular Exit Pupil
Exit pupil is a number that indicates how bright an object will appear when viewed in low-light situations. A higher number means brighter images. A large exit pupil also makes it easier to maintain a full image of an object if your hands move or shake.
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.
In very dim light, our pupils can widen up to 7mm. If your binoculars have an exit pupil size of less than 7, then they are restricting the light available to your eyes. 7 x 50 binoculars offer an exit pupil size of 7.1mm—a good choice for nighttime viewing.
For low-light situations (dawn, dusk, within dense tree cover or while observing the night sky), models with a high exit pupil number (about 5mm or higher) are good options.
For 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.
Binocular Relative Brightness
Related to exit pupil, relative brightness also measures how bright an object appears to your eyes: the higher the number, the brighter objects will appear. This is useful in low-light situations.
Relative brightness is determined by squaring the exit pupil number.
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.
Binocular Eye Relief
This is the distance between each eyepiece and your eyes while the whole field of view is visible. Long eye relief increases comfort by allowing you to hold the binoculars away from your face.
The eye-relief spec is most useful if you wear glasses. Most manufacturers recommend that glasses wearers should roll down the rubber eyepiece collars before viewing; some exceptions do exist.
Tip: If you wear glasses, look for eye relief of 11mm or more.
Binocular 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.
Almost all binoculars feature a central focusing wheel that focuses both barrels on the binoculars at the same time. They also typically include a diopter adjustment ring that focuses one barrel independently of the other. This allows you to compensate for differences in vision between your two eyes. The diopter ring is usually located on either the left or right barrel near the eyepiece.
Binocular Prism Type
Binoculars are made with different style prisms inside. Without prisms, everything you look at would appear upside down due to how light rays pass through the lenses that are inside binoculars. There are two styles of binocular prisms:
Porro: Binoculars with Porro prisms provide good optics, but they are bulkier than binoculars with roof prisms. These binoculars tend to be less expensive because they're easier to produce.
Roof: These binoculars tend to be slim and compact, making them a good choice for hiking and other outdoor activities. It takes a bit more precision to make binoculars with roof prisms, so these are typically more expensive than those with Porro prisms.
Binocular Lens Coatings
Some of the light that passes through the lenses in binoculars is reflected away. This reflection reduces the amount of light passing through the lenses and causes the image to appear dark. To reduce reflection and ensure clear, sharp images, coatings are applied. Fully multicoated lenses reduce the most reflection and increase light transmission.
Waterproof and Weather-Resistant Binoculars
If you’ll be using your binoculars aboard a boat or on land during a rainy day, you’ll want to consider waterproof or weather-resistant binoculars.
Waterproof binoculars typically use O-rings to create a seal to prevent moisture from entering. Waterproof binoculars also prevent dust or small debris from getting in.
Weather-resistant binoculars are not fully waterproof. They are designed to protect against light rain but not submersion.
Binoculars can be prone to fogging up when you move them between different temperatures, such as from the cold outdoors to the warmth of your home. Fogging is not only annoying, but can also be potentially damaging if moisture gets trapped inside.
To counter fog, binocular makers have developed methods for replacing the air inside the optical barrels with inert gas that has no moisture content and therefore won’t condense. This protects against fogging up of the internal lens surfaces, not the exterior ones.
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