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Binocular Terminology

What do the numbers mean?

Binoculars are normally identified by two numbers. The first number is the magnification and the second number is the diameter of the objective lens in millimeters (mm). For example, 7x35 means the binocular magnifies objects 7 times and the objective lens is 35 mm in diameter. A 10x50 binocular magnifies objects 10 times and has an objective lens diameter of 50mm.


There are generally two categories of prisms, porro prisms and roof prisms. Porro and roof prism binoculars are distinguished by their shape. Porro prism binoculars tend to be larger and have a definite offset to their body design. Roof prism binoculars are more straight and streamlined. Each provide excellent viewing.

Field of View

Describes the size of the area that can be seen through the binocular or spotting scope. Expressed as the number of feet per 1000 yards of distance, the higher the magnification the narrower the field of view. On wide angle binoculars (designated WA on Alpen binoculars) a special lens design allows a wider field of view than normal. An example field of view for a standard 7X binocular would be 372 feet, whereas a wide angle would be 487 feet. Wider field of views are often preferred by hunters who want to scan large areas for game or for viewing sporting events.

Objective Lens Diameter

This is important because many of the performance characteristics of a binocular are determined by the diameter of this lens. The function of the objective lens is to collect light and direct it to the prisms or eye pieces. The larger the diameter, the more light will reach your eye thus providing a brighter and sharper image. However, the larger the diameter, the heavier and bulkier the binocular will be. You will find there are many different combinations of magnification and objective lens diameters to meet specific applications.

Eye Relief

Eye Relief is the distance, expressed in millimeters, from the ocular (eyepiece) lens to the point where the eye is positioned to view the entire image. Eye relief is affected by magnification, the number of lens elements, and field of view. Binoculars designed with long eye relief are usually preferred by those who wear eyeglasses. Most binoculars provide 8-13mm of eye relief. Binoculars with "long eye relief" provide from 14-20mm (or more) eye relief.

Exit Pupil

A measurement of how much light is usable by a binocular or spotting scope. Generally, the larger the exit pupil, the brighter the image will appear, particularly in dim light. Exit pupil is determined by dividing the diameter of the objective lens by the magnification. For example, a 7x35 binocular has an exit pupil of 5 mm. A 10x50 also has an exit pupil of 5 mm, so these are about equal in dim light. A 7x50 binocular however has an exit pupil of 7 mm. For general use in bright light, an exit pupil of 2.5 mm is sufficient and 4 mm and higher generally allow better viewing in dim light. Lens coatings and the human eye also affect how well a binocular performs in dim light.


Describes how sharp or clear an object appears when viewed. Resolution is primarily determined by the objective lens diameter. The larger the diameter of the objective lens, the better the resolution. However, lens coatings, lens alignment, quality of glass, and quality of prisms also affect how sharp a viewed image will be.

Coated Optics

Quality optics are coated to prevent glass surfaces from reflecting light and loosing it. The very best optics are "Fully Multi-Coated" which means multiple coatings are layered fully onto all glass surfaces. "Fully coated" means all "air to glass" surfaces have been coated with at least one layer of coating. "Coated" means some surfaces have been coated.