Definitions of Binocular Terms


A binocular consists of two small telescopes that are joined by a hinge and usually share a common focusing mechanism. They have the ability to create a single image for both eyes simultaneously while providing a realistic perception of depth. Using prisms, binoculars are designed to give a correctly oriented, right side up view of an object in the distance. A good pair of binoculars is one of the handiest accessories for the birdwatcher, nature lover, astronomer, sportsman, traveler, and hunter. So whether you want to find a bird in the woods or get a better view of the local football game, a good pair of binoculars is your best companion. Binoculars are available in a great variety of sizes with magnifying powers and features to suit any purpose, preference, or budget. Here are some definitions and descriptions of common binocular features to help you better understand binoculars and make an informed decision when making your purchase.

Ocular Lens

The term ocular lens is derived from the Latin word ocularis, meaning eye. On the binocular, this is the lens closest to the eye; it is also sometimes called the rear lens.

Objective Lens

The objective lens, or front lens, is on the end of the binocular closest to the object being viewed.

Binocular Identification Numbers

Binoculars are often referred to by two numbers separated by an "x", for example, 8 x 32. The first number is the power of magnification of the binocular. The second number is the objective lens size.

Magnification (Power)

Example: 8 x 32 binoculars have a magnification power of 8.

With an 8 x 32 binocular, the object being viewed appears to be eight times closer than you would see it with the unaided eye. For example, if you view a bird perched 200 yards away from you through 8x binoculars, it will appear as though it were 25 yards away (200 divided by 8).

So, the greater the magnification or 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 Size

Example: 8 x 32 binoculars have objective lenses measuring 32mm in diameter.

The diameter of the objective lenses determines how much light is able to enter your binoculars. Comparing an 8 x 32 binocular with an 8 x 40 binocular with all other features identical, more light is captured by the binocular with 40mm diameter lens. More light means a brighter image, particularly in low-light conditions.

ArmorTek Lens Coating

Ultra-hard, scratch-resistant coating protects exterior lenses from scratches, oil and dirt.

Interpupillary Distance (IDP)

Interpupillary distanceis the distance between the centers of the pupils of the two eyes. IPD is critical for the design of binocular viewing systems, where both eye pupils need to be positioned within the exit pupils of the viewing system.

Exit Pupil

Exit pupil is a number that indicates how bright objects will appear when viewed in low-light situations. The exit pupil is the bright circle that can be seen in the center of each eyepiece when you hold the binoculars about a foot away from your eyes with the objective lenses pointed toward a bright light. The larger the diameter is, the brighter the view field is. This is an important consideration when you plan to use your binoculars in dark situations and/or for astronomical observation.

Formula for determining the exit pupil:

The effective diameter (size) of the Objective lens ÷ Magnification = the Exit pupil

With 8 x 42 binoculars, the formula is 42 ÷ 8 = 5.3.
Therefore, the diameter of the exit pupil is 5.3mm.

Apochromatic objective lenses (APO) are capable of producing the sharpest, crispest images in the widest-field optically possible. Each color has a different wave length and as they pass through any lens or piece of glass, they focus at different distances from the lens. This effect produces color fringing at high contrast edges resulting in softer images. The Apochromatic lenses are designed with three elements and bring the light of three different wavelengths (usually red, blue and green) to a common focal plane. The result is improved resolution, particularly at the edge of the image field, as well as an extraordinarily high-contrast image. Spotting birds in the forest has just been made much easier. These birds come to life through these binoculars. Similar comparison between HD TV and standard TV. The APO technology is high priced and usually found in only in the top of line optics.

XD Lens Elements

Extra-low dispersion (XD) glass increases resolution and color fidelity, resulting in crisp, sharp images.

HD Lens Elements

HD (High Density) extra-low dispersion glass delivers impressive resolution and color fidelity, resulting in High Definition images.

Prism Systems

The prism system of a binocular reduces the size needed to contain a long optical path, compared to that of a telescope, and turns what would be an upside-down image right-side-up. There are two types of prism systems, Porro prism and Dach prism (commonly referred to as Roof prism).

Porro Prism System

Porro prism binoculars can be identified by their offset barrels; the objective lens is not in line with the ocular lens. The typical Porro prism design consists of two right-angled prisms joined so as to reflect the light path 3 times, resulting in a corrected image which is usually optically superior to the roof prism design. Porro prism binoculars provide a greater depth perception and generally offer a wider field-of-view. 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. Because of the simplicity of this system, some of the best values can be with a Porro design.

Dach (or Roof) Prism

Roof prism binoculars can be identified by their straight barrels; the objective lens lines up directly with the ocular lens. In the Roof prism design, the prisms overlap closely, allowing for a more streamlined shape. The Roof prism design is technically more complicated than the Porro prism design, with prisms requiring very precise angles and polishing. The light path is reflected 4 times. This design is more compact, but as the light path is longer and the prisms more complicated, the glass and manufacturing need to be of higher quality than the Porro prism design for equal performance. These binoculars usually have two pivot points between the tubes, and are more difficult to adjust to the spacing of your eyes. Roof prism binoculars are less bulky and more rugged than an equivalent Porro model. To be really good, roof prism binoculars have to be in the high price class. Do not attempt to economize on roof prism binoculars.

Prism Glass

Most optical prisms are made from borosilicate (BK-7) glass or barium crown (BAK-4) glass. BK-7 prisms are satisfactory and usually found in lower priced binoculars, but they are inferior to the BAK-4 prisms. BAK-4 is made of superior optical glass producing clearer, brighter images and high edge-to-edge sharpness.

Phase correction

Phase correction is a set of coatings on the prism glass of high quality roof prism binoculars that keeps light in correct color phases. These coatings are only needed on roof prism binoculars to enhance resolution, contrast, and color fidelity.

Dielectric Prism Coatings

Multi-layer prism coatings increase reflection which provides bright, clear, color-accurate images; most noticeable in low light conditions.

Plasma Tech is cutting edge technology for applying multiple coatings of over 10 layers to high-end lenses. This process provides light transmission of 99.9% over a range that extends from ultraviolet to near-infrared light.

Coated Optics

Lens surface coatings reduce light loss and glare due to reflection for a brighter, higher-contrast image with less eyestrain. Multi-coated lenses improve image contrast and clarity, for the most accurate, detailed viewing possible. Review carefully the description of the lens coating of the optics. Fully Multi-Coated lens will provide the best viewing experience.

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.

XR Fully Multi-Coated - Proprietary multiple anti-reflective layers on all air-to-glass surfaces.

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

Adjustable Eyecups

Adjustable eyecups allow users, with or without glasses, to use the same binocular. There two variations of eyecups:

Twist eyecups provide precise, intermediate settings to maximize custom fit for comfortable viewing with or without eyeglasses by twisting the eyecups up and down.

Roll-down or Fold-down rubber eyecups provide custom eye relief for comfortable viewing with or without eyeglasses, some when left extended block light from the sides while viewing.


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

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. If you wear glasses, look for eye relief of 11mm or more.

Eyeglass Wearers - Eyecups

For additional viewing comfort and flexibility, many binoculars offer rubber eyecups which can be folded down to reduce your distance from the eyepiece lens. Some even have a turn-and-slide mechanism which extends or retracts the eyecups smoothly.

Field of View

Field of view measurement indicates the width of the area you see through your binoculars. It is easier to spot things without moving your binoculars with a wide field of view. You will find this useful for watching birds, or following an athlete on a playing field. When field of view is measured in feet, it is measured as the width of your view at 1,000 yards away. In general, most good-quality binoculars have a field of view between 300 and 375 feet at 1,000 yards.

Field of view can also be measured in degrees. Most binoculars have a field of view between 6 and 7.5 degrees.


O-ring seals prevent moisture, dust and debris from penetrating the binocular for reliable performance in all environments.


Lens fogging can be caused by using binoculars or any optics in foggy, misty, rainy, and/or high humidity conditions. This can also occur with rapid temperature change, such as bringing out binoculars from an air conditioned building or car into a hot and humid environment. If you will be using your binoculars in any of these conditions, then you should consider Fogproof binoculars.

Gas purging is the process of removing oxygen from the barrels of the binoculars, and therefore from in between the optics, during the manufacturing process and either nitrogen gas or argon gas is used in its place. These gases are stable (inert) and don't easily react with other elements. This characteristic is what keeps condensation off of the internal optics.

Nitrogen gas purging is a traditional, low cost manufacturing process that delivers fogproof, waterproof performance by preventing moisture from the inner optics.

Argon gas purging is a newer, more expensive process that delivers fogproof, waterproof performance by preventing moisture from the inner optics.

Argument: Argon molecules are larger than nitrogen and thus less likely to leak out beyond the O-ring seals after the binoculars experience a hard fall. It is also considered to prevent moisture over a wide range of temperatures. Argon purging is one of the highest quality water proof and internal fog proof methods available.

Searches on the internet have found this argument hotly debated. Most people seem to believe that the quality of the O-rings and material used in the casing are more important factors in the quality of the fog protection than use of nitrogen vs. argon gas.


Some binoculars are O-ring sealed and nitrogen-purged for total waterproof and fogproof protection. Some of these models can withstand complete immersion in water and stay dry inside. Check the product information for details. The interior optical surfaces won't fog due to rapid temperature change or humidity.

Rubber Armor

Rubber armor protects your binoculars in many ways. The armor suppresses the noise level when the binoculars bump into other metal objects that would otherwise spook wildlife. It helps protect the binocular from the bumps and scratches that come with day-to-day use. The gripping surface is more comfortable and secure, reducing the risk of dropping the binoculars. Cleanup is a breeze after a tough day in the field.

Open Hinge

The open hinge binocular is a cut - away design that reduces weight making these binoculars less tiring to hold for prolonged periods of time.

Image stabilization

Hand motion can cause the binoculars to shake and vibrate making for a difficult experience. The image stabilization technology is designed to reduce these shakes and vibrations. Built-in sensors detect movement when a shake occurs; a tiny microprocessor adjusts the binoculars' optics to compensate for the movement. The result is a steady, consistent view. The benefits will be observed by birdwatchers attempting to follow a bird's movements through the treetops, people trying to get a clear view of a distant object while aboard a moving boat, fans watching the action at a sporting event, etc.

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