Binocular Glossary

by Richard J. Bartlett

When you’re shopping for binoculars, it’s not simply a case of trying to get the most magnification for the least amount of money. It’s important to know what to look for when choosing the right binoculars, and you need to understand the basic terms you’re likely to come across.

For example, all binoculars have two numbers associated with them. You might see something like 7×35 or 10×50 – but what do those numbers actually mean? The first indicates the magnification, while the second is the aperture, as measured in millimeters.

Most of us are familiar with magnification, but what about aperture? Is it important? Is a smaller or larger aperture best? This article will answer those questions.


This is the diameter of each objective lens (the large lenses at the end of the binoculars), as measured in millimeters.  Objective lenses gather light, so the larger the lenses, the more light can be gathered, which is important if you’ll be using the binoculars in low-light conditions. For example, night-time hunters and astronomers will want to use larger aperture binoculars. 


The downside is that the larger the aperture, the heavier the binoculars. Objective lenses typically range from 20mm to 100mm in aperture, but many users prefer an aperture of 50mm at most.

Exit Pupil

This is the image formed by binoculars as it exits the eyepieces as a beam of light. Measured in millimeters, the exit pupil of binoculars indicates how wide that beam of light is. For example, if your binoculars have an exit pupil of 7mm, then the beam of light exiting the eyepieces is 7mm wide.


This can be important to know for two reasons: firstly, the larger the exit pupil, the brighter the image under low-light conditions. 


Secondly, while a larger exit pupil sounds like a good idea, it’s worth knowing that it will also depend upon how much your eyes can dilate. Everyone is different, but most people’s eyes will dilate more when they’re younger, thereby allowing their eyes to gather more light.


While dilation can range between 4mm and 9mm, it’s generally accepted that 7mm is a good average for most people, and you’ll want binoculars that stay within the 5mm to 7mm range. If the exit pupil is too small, the image may not be bright enough. If the exit pupil is too large, the image could be too bright during the daytime and your eye may not be able to dilate enough to see the entire image.


In case you’re wondering, it’s easy enough to calculate the exit pupil of your binoculars – simply divide the aperture by the magnification. For example, 10×50 binoculars have an exit pupil of 5mm. If you need to know how far your eyes can dilate, you’ll need to see an eye doctor!

Eye Relief

In short, eye relief is the distance from the eyepieces that the exit pupil is formed. For example, binoculars with an eye relief of 17mm form an image 17mm away from the eyepieces. This can be important to know if you wear glasses, as you won’t be able to hold the binoculars directly up to your eyes, but rather a short distance away from them instead. Hold the binoculars too close and you may not see the whole image. Hold the binoculars too far away, and the image might appear too small.


Anti-reflective coatings are applied to the optical elements of binoculars – specifically, the lenses and prisms – in order to improve the light transmission rate and the quality of the image. However, manufacturers may use their own proprietary coating and not all binoculars will have the same number of coatings applied.


The vast majority of manufacturers will produce binoculars with optics that are either multi-coated (MC) or fully multi-coated (FMC):

Multi-coated (MC) means that at least one optical element has been coated multiple times on at least one side. Again, the exact number of optics and coatings will vary by manufacturer.

Fully multi-coated (FMC) means that all the optical elements have been coated multiple times on both sides.


Where the optics are either multi-coated or fully multi-coated, manufacturers will typically apply between 5 and 7 layers to the glass. Since the coatings are designed to reduce glare, it stands to reason that fully multi-coated optics will produce a better quality image, with fully multi-coated optics having a light transmission rate of between 90% and 95%.


You’re probably already familiar with porro prism binoculars. These are the traditional style of binoculars that have barrels slightly offset from the eyepieces, giving the binoculars a W shape. However, in recent years manufacturers have started to produce roof prism binoculars. These have barrels that are in-line with the eyepieces, giving them an H shape.


Roof prism binoculars are designed to be more lightweight and compact than their older porro prism cousins, but there are a few trade-offs. They may be smaller and lighter, but porro prism binoculars are a little cheaper to manufacture and typically produce a better quality image. This is partly because the objective lenses on porro prism binoculars are further apart, allowing you to have larger aperture lenses as a result. Since larger aperture lenses can gather more light, the image quality is usually a little better. 


The eyepieces are the smaller lenses you look through.

Interpupillary Distance (IPD)

The interpupillary distance (IPD) is the distance between the pupils of your eyes. Measured in millimeters, it corresponds to the distance between the two eyepieces of binoculars and is specified as an adjustable range. By moving the barrels of the binoculars closer together or further apart, you can adjust the distance between the eyepieces to match the distance between your eyes.


As many folks are aware, the magnification indicates how much larger your target will appear through the eyepieces. Most magnifications will range from 6x to 20x, but it’s also possible to buy binoculars with a magnification of 25x. Be wary of binoculars that feature a zoom magnification, as they tend to produce a lower quality image.

Objective Lenses

The objective lenses are the large lenses you point toward your target. (See aperture for more information.)


The prisms in binoculars have two roles: firstly, they flip the image horizontally and vertically, so that it appears correctly oriented. Without prisms, the view would appear both upside-down and back-to-front. Secondly, they also refract the light so that it takes the shortest path from the objective lenses to the eyepieces. This allows binoculars to be more compact and portable than telescopes.


There are two types of binocular prism in common usage today: Bk-7 and BaK-4. Of the two, BaK-4 prisms refract more light, resulting in a clearer, brighter image. However, as you might expect, BaK-4s cost more to produce, so many of the more inexpensive models will use Bk-7 prisms instead.