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Glossary of Binocular Terms
written by Bushnell

Glossary of terms to help you understand binocular technology.
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Roof Prism Binos
Roof Prism System
Porro Prism Binoculars
Porro Prism System

Diopter Setting.
Diopter Setting

Roll-Down Eye Cups
Roll-Down Eye Cups

Prism Systems

The prism system of a binocular reduces the size needed to contain a long optical path and turns what would be an upside-down image right-side-up. There are two types of prism systems: roof and porro.

Roof Prism 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, streamlined shape in which the lenses and prisms are in a straight line. Roof prism binoculars are less bulky and more rugged than an equivalent porro model.

Porro Prism System

In porro prism binoculars, the objective or front lens is offset from the eyepiece. Porro prism binoculars provide a greater depth perception and generally offer a wider field-of-view. Because of the simplicity of this system, some of the best values can be with a porro design.

Waterproof/Fogproof

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.

Magnification (Power)

Binoculars are often referred to by two numbers separated by an "x". For example, 8x32. The first number is the power of 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.

Objective Lens Size

The second number in the formula (8x32) is the diameter of the objective front lens. The larger the objective lens, the more light that enters the binocular and the brighter the image.

Prism Glass

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-to-edge sharpness.

Coated Optics

Lens surface coatings reduce light loss and glare due to reflection for a brighter, higher-contrast image with less eyestrain.

Types of Coatings

Coated - A single layer on at least one lens surface.
Fully Coated - A single layer on all air-to-glass surfaces.
Multi-Coated - Multiple layers on at least one lens surface.
Fully Multi-Coated - Multiple layers on all air-to-glass surfaces.

Field-of-View (FOV)

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. Generally, the higher the magnification, the narrower the field-of-view.

Resolution

Resolution, or definition, is the ability of a binocular to distinguish fine detail and retain clarity.

Exit Pupil

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).

Eye Relief

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.

Eyeglass Wearers - Eyecups

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.

Diopter Adjustment

A "fine focus" adjustment ring usually provided around one eyepiece to accommodate for vision differences between the right and left eyes.

Rubber Armor

Rubber armor provides multiple benefits. It helps protect the binocular from the bumps and scratches that come with day-to-day use. It provides a comfortable gripping surface for making them easier to hold on to. It's easy to wipe clean after a tough day in the field. And it suppresses noise if the binocular bumps aluminum or other non-rubber surfaces, which might otherwise spook wildlife.

RainGuard HD

Hydrophobic (water-repellent) coating on which condensation from rain, fog or snow forms in much smaller droplets than on standard coatings. Smaller droplets scatter less light, which results in increased light transmission and a clearer image. Makes the binocular useful even when looking directly into the driving rain. (Bushnell optics exclusive)

PC-3 Phase Coating


Found on the best roof prism binoculars, this chemical coating is applied to the prisms to enhance resolution and contrast. Would not provide an advantage on porro prism models. (Bushnell optics exclusive)

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