Trying to pick out binoculars can be a confusing task! There are so many numbers, terms and types from which to choose. It's all about image clarity when choosing binoculars, so in this guide, I will define magnification and objective lens (written as 8x40mm. 8x being the magnification and 40mm being the objective lens), the exit pupil (measured in millimeters, or mm), as well as the coatings and prisms used in binoculars.
The first thing you should consider when buying binoculars is the magnification. This will be a number followed by an x. The magnification is the measurement of how much bigger the object you are looking at will appear to be. It will also make the object appear that much closer to you. 8x magnification is excellent for viewing game. Nikon's Action EX binoculars are a good example. Keep in mind, the higher the magnification, the lower the image brightness, the less clarity of the image, and the smaller field of view. Anything above a 10x magnification will probably be too high for hunting purposes. These higher magnification binoculars are generally used with a tripod and are not hand-held.
The second number we look at is the objective lens or the "aperture". The diameter of the objective lens (front lens) is measured in millimeters. If you want binoculars for hunting or birding you'll want to look for a binocular with a smaller magnification and a larger objective lens, like Bushnell's 8x42mm Legend binoculars. The objective lens determines how much light can pass through. The bigger the objective lens, the more light can get through. The disadvantage in a larger size objective lens is that the binoculars get heavier to hold.
Another important number you should consider is the field of view (F.O.V.). The field of view is the side-to-side measurement of the circular viewing field or subject area. The smaller the field of view, the harder it will be to spot your target. Field of view is usually measured in feet, such as 330' field of view at 1,000 yards. In other words, if you are looking through binoculars at a target that is 1,000 yards away, you will have a 330' wide view.
The exit pupil size is another number that is important when picking out your binoculars. The exit pupil size is related to image brightness. The larger the exit pupil number, the brighter the image, especially in low-light situations. Note that anything larger than a 7mm exit pupil is useless. 2.5mm to 3mm exit pupil is fine for normal viewing. For example, Leupold's 8x30mm Wind River Yosemite binoculars have a 3.8mm exit pupil. If you want to view things in lower light, look for a higher exit pupil number, such as the 4.2mm exit pupil in Bushnell's 10x42mm Legend Binoculars. To determine the size of the exit pupil, divide the objective lens diameter by the magnification power (a 8x32 model has an exit pupil of 4mm).
All binoculars lenses will be coated. The different coatings are relative to the quality of the image. For the best quality, look for fully multi-coated lenses, such as Bushnell's 10x50mm Powerview Binoculars. Multi-layered/coated coatings increase light transmission and improve image quality.
Another term you will see in binocular descriptions is the type of prism used. Without a prism, you would see the image upside down! The Porro prism is cheaper and bulkier than the roof prism. Roof prisms are smaller and allow for smoother focusing. The prism will also be described as a BK7 or a BAK4. BAK4 is better quality, like Alpen's 10x42mm Apex binoculars.
In summary, here is a short glossary of terms you should remember when choosing binoculars:
- Magnification - the number which shows how much bigger the object that you are looking at will appear to be.
- Objective lens - the lenses at the front of the binoculars which allow light to pass-through
- Exit pupil - the number that is related to image brightness
- Prism - the glass used in binoculars to make a correct, upright image
- Fully coated - all glass surfaces that are exposed to air are coated with a single layer
- Multi-coated - allows for the maximum amount of light to pass through
- Center focus - Uses a single wheel to focus
- Diopter adjustment - lets you adjust for each eye
- Eye relief - the maximum distance your eye can be from the eyepiece and still see the entire field of view
- Field of View - width of the sight picture
- Close focus - the minimum distance an object must be to view it (for birding, you need at least a 4m close focus distance.)