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Binocular Guide
Binoculars are available in a wide range powers, sizes and features for different uses. Binocular equipped with two optical systems that are attached by a hinge sharing a common focus system. An image being viewed through a binocular can be projected at the same time through both eyes providing a realistic perception of depth.
  Commonly Used Phrases and Terms

The prism system of a binocular reduces the size of a long optical path and correct an inverted image. There are three most common types of prism construction.

Roof Prism System
The prisms overlap and are aligned in a straight line with the lenses allowing to have a slim construction.

Porro Prism System
The objective is offset from the eyepiece offering greater depth perception and a wider field of view.

Reverse Porro System
The objectives are offset from the eyepiece in a horizontal way reducing the total size of the construction by about 30% compared to the traditional porro prism system.

The common standards of optical prisms are barium-crown (BAK-4) glass or borosilicate (BK-7) glass.  The BAK- 4  is a higher quality glass that reduces light scattering, resulting in sharper and brighter images.

Magnification is the number of times the object being viewed is enlarged. Binoculars are often identified by two numbers, for example: 10×50. The first number indicates the magnification or the power of the binocular, expressed by the letter “x”, for times. This means that the object being viewed appears to be 10 times closer than would be seen with the naked eye.

The second number in the reference (i.e. 10X50) indicates the diameter of the objective lens or the front lens. The larger the objective lens, the better the light transmission, thus the brighter the image.


Wide Angle
Binoculars with a wider field of view. This wide-angle feature is better for capturing action or fast sports.

Zoom binoculars have variable powers of magnification allowing you to view closer in the distance without changing binoculars.

Waterproof binoculars are O-ring sealed for complete waterproof protection to keep dry inside after immersion in water. Some binoculars are also fogproof which means that they are nitrogen-purged to prevent anti-fogging inside the optical surfaces.

Various coating processes on the lenses and prisms will determine the brightness and the light gathering of a binocular, providing higher contrast and brighter images. The different types of coating are:

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

This is the side-to-side linear measurement of the of the circular field seen through a binocular. It is defined by the width in feet of the area visible at 1000 yards, or in meters at 1000m. The higher the magnification, the narrower the field of view.

The closest distance to the observed object that the binoculars can be used while retaining a sharp focus.

The measurement of the binocular’s ability to distinguish fine detail and sharpness.

This refers to the size of the small disc of light visible at the eyepiece of a binocular. To determine the size of the exit pupil, divide the objective diameter by the power: For example,  a 10×50 binocular will have an exit pupil of 5mm

This is the distance a binocular can be held away from the human eye and can still observe the entire field of view. Long Eye Relief (LER) reduces eyestrain and is more comfortable for eyeglass wearers.

The mechanism that is used to focus both barrels of a binocular on the object being viewed. They are normally three types of focus systems.

Center Focus – Where the focusing knob is located in the center of the binocular
Individual Focus – Where the focusing knob is located individually in each eyepiece
Fixed focus – No focusing required

A ring adjustment for fine-focusing which is usually provided around one eyepiece. This allows to compensate for vision difference between the right and left eye.

The most common eyecups are twist-up, pop-up, or rubber fold-down