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Sport Optics Guides

Binoculars 101 - Complete Guide To Optics

Binoculars 101 - Complete Guide To Optics

Binoculars are a handheld optical instrument composed of two telescopes and a focusing device. Typically used to increase magnifying ability.

The primary things to know about a binocular is magnification, aperture, exit pupil, twilight factor, size and weight, and the field of view.


Magnification is how many times the image appears to be magnified. Typically 8X, 10X, 12X, 15X etc. When you see a pair of binoculars listed as 8x42, the first number "8" is the magnification. The magnification needed depends on the use and topography the binoculars will be used in. The most popular size is 10X because of its versatility. A user who is more concerned with field of view should consider a lower magnification such as 8X while a user more concerned with long distance viewing should consider a 12X or 15X. The higher the magnification, the more perceived shaking the user will see in the image. Whenever possible, attach your binoculars to a tripod via an adapter for a better viewing experience.

Aperture (Or Objective)

Represents the diameter of the objective lens (the lens furthest away from you) in millimeters. The size of the objective lens along with the power of the magnification determines the light gathering capability. 

Exit Pupil

The size of the beam of light that comes from the binocular into your eye. It is calculated by dividing the size of the objective lens by the magnification. For example, an 8x32 has an exit pupil of 4 (32/8=4). 

Twilight Factor

Is an estimation of how much detail can be seen in low light. It is calculated by multiplying the Magnification by the size of the Objective then by taking the square root of that number. For example, an 8x32 has a twilight factor of factor of 16 (8x32=256 and the square root of 256=16).

Size & Weight

Magnification and size typically go hand in hand but size is not always determined by magnification. Smaller objectives such as a 32mm are going to be offered in 8X or 10X and be much more compact than a higher magnification (12X or 15X) with a larger objective such as 56mm.

Field of View

How wide of an area is encompassed in a binoculars' image? It is expressed as either a degree or as the width in feet at 1000 yards.

Additional Binocular Considerations


Do I need a rangefinder in my binoculars? For hunters and shooters, the answer is yes.


The glass is important, but the coatings on the glass is what makes the biggest difference in clarity. Coatings help prevent fogging and ensure color clarity and contrast.


Use air to blow debris off your lenses. Avoid touching the lenses of your binoculars with anything other than a microfiber cloth or lens pen. When you use your shirt to wipe the lens you are actually removing the coatings which over time will degrade the image. 

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