Understanding Binocular Terminology

So you’ve decided to purchase a pair of binoculars. That’s fantastic! Familiarizing yourself with some basic binocular terms will help you make a more educated and appropriate choice.


8×25, 8×42, 10×42 etc.

Pronounced “8 by 25”, and so on, these are the critical numbers used to describe a binocular. The first number is the magnification (eg. an 8x bino makes an object appear 8 times larger or closer than it actually is). The second number refers to the diameter (size) of the objective lens (the lens opposite the eyepiece) in millimeters. The larger the number, the larger the objective lens, the more light is gathered, and the brighter the image.


Magnification. Usually 8x or 10x. Indicates how many times larger (or closer) an object appears.


Contains the lens (the ocular lens) you look into to view images through the binocular.

Objective Lens

The lens closest to the subject you are looking at (the furthest from your eyes). The size is measured in millimeters and is the second number used to describe binoculars (see above).

Close Focus

How close the binoculars can focus on an object. Usually you will want a close focus of 6-12 feet. If you are observing smaller birds or butterflies close up, this can be quite an important consideration.

Field of View

The area that can be seen when looking through a pair of binoculars. A larger field of view makes it easier to locate the subject. Usually binos with higher magnification have reduced field of view. The trade-off between magnification and field of view is an important decision point.

Focus Knob

This is turned to bring the image in and out of focus. Typically located on top in the center of the binocular.

Eye Relief

The distance from the outer surfaces of the eyepiece lenses to the position of your eyes to obtain a full field of view. If the distance is too short, your eyes or eyelashes may touch the lens. But if it’s too great, your field of view is diminished.


Adjusts the focus of the right optical barrel to compensate for differences in clarity and focus between your eyes. It is usually an adjustable ring on the right eyepiece or the central focus wheel.

Center Hinge

The movable part that allows you to adjust the distance between the two barrels.

Roof vs Porro Prism

The basic type or style of binocular. Porros have a right-angled bend and look a little M-shaped (like this Vortex Raptor model, for example). They are the shape most people think of when they think of binos. Roof prism models look more like an “H” (like this Vortex Diamondback model). Roof prism models are more compact and, due to advanced lens coatings and other advancements, are the design most commonly used in the industry today.


Top Image Credit: By Alex Proimos from Sydney, Australia – Bird Spotting, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=25648340


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