To look at, one pair of binoculars would appear to be much the same as another. Some might be smaller than others but, by and large, they’re made up of a pair of barrels with lenses at either end, attached to each other with a hinge in the middle.

Dig deeper, however, and you’ll discover there’s a whole world of features and specifications that determine how good a pair of binoculars are, and the sort of spotting activities they’re suited to.

You’ll probably have noticed if you’ve ever shopped for or used a pair of binoculars, that they all have a pair of numbers, like 8 x 42, or 10 x 25, written on them.

Magnification

The magnification specifies the factor by which an object appears to be closer in comparison with the actual distance. The higher the magnification, the closer the object seems to be. However, a higher magnification also means a smaller field of view. Check the precise product name as the number in front of the ‘x’ specifies the magnification. For example, 10x42 is a device with 10x magnification. 

The first number designates the magnification level: an 8x pair will enlarge your subject eight times while a 10x pair will make it ten times bigger. The second number tells you how large the objective lenses (the big ones on the end) are in millimetres.

The most important number is the magnification level. You might think here that bigger is better, right? Well, that’s not strictly true. While a 10x or 12x magnification will allow you see things further away that bit closer, higher magnifications such as these can bring their own down sides. At higher levels of magnification, it's hard to hold binoculars steady enough to see a stable image. A higher magnification also usually means a narrower field of view. Field of view (or FOV), incidentally, is another number on the binocular somewhere. It's normally expressed in degrees, and it refers to how much you can see from left to right when you’re peering through them.

The following information aims to give you a basic understanding of how binoculars work, the different specifications available and what they mean, plus some points to help you choose the best instrument suited to you.

Specification Example: 8x42. The number ‘8’ denotes the magnification and means an object appears to be 1/8th of its actual distance away. Using this rule an object 40m distant appears 5m away. ‘42’ is the diameter of the objective lens (the large end) in mm through which light enters the binocular.

For birdwatching and nature-spotting, most people go for 8x. This gives you a reasonably broad field of view (usually around 7 to 8 degrees) and magnification and allows you to hand-hold easily without getting too much distracting shake. If you’re more of a stargazer and need binoculars for looking at the moon and constellations, you’ll want a higher magnification level, 12x and up, but you'll need to think about attaching them to a tripod.

Field of view

The field of view describes the size of the image section that can be seen through the optics. This is specified either in meters (width) at a distance of 1000 meters (m/1000m), or feet (width) at a distance of 1000 yards (ft/1000 yds), or as an angle (degrees). The higher the magnification, the smaller the field of view.
Binoculars have a large field of view, which means you can see a wide area. Spotting scopes have a higher magnification, which makes the field of view much smaller, but you can see more detail.

Objective lens diameter

The objective lens diameter specifies how much light can enter the optics. This makes it a key factor in an instrument’s performance, for example, in twilight. The bigger the objective lens diameter, the more light the objective lens can capture. The darker the surroundings, the larger the objective lens diameter needs to be. Check the precise product name as the number after the ‘x’ specifies the objective lens diameter in millimeters. For example, a device with the suffix 10x42 has an objective lens with a diameter of 42 mm.

Shortest focussing distance

The shortest focusing distance specifies how close an object needs to be to see it clearly with the optics. Between this value and infinity, it is possible to focus the image.

Roof prism binoculars

The popularity of roof prism binoculars is a result of this system being favoured in the development of instruments with user oriented features such as waterproofing, long eye relief and close focus.

What magnification? The higher the magnification relative to the objective lens size, the lower the brightness and the shallower the depth of focus (distance in focus at a single focus setting.) For general observation choose 7x or 8x. If you want 10x or more try them first as magnification amplifies hand-shake affecting image stability.

What objective lens size? The amount of light entering a binocular is related to the surface area of the objective lens ‘OG’. A 50mm OG will admit 2.5x the light of a 30mm OG. The amount of light reaching the eye is called the exit pupil diameter ‘EPD’ and its size can be found by dividing the OG diameter by the magnification. For example the EPD of an 8x32 = 4mm while the EPD of an 8x56 = 7mm. As a general rule your iris dilates 2-3mm in bright sunlight, up to 6-7mm in twilight. With an EPD of 5-6mm 7/8x42’s and 10x50’s will outperform 8x32’s and 10x42’s but are larger and heavier for their given magnification. 

The field of view of a binocular is dependent on the optical design and is expressed as either the width of panoramic view in metres from a distance of 1000m or in degrees where 1° is approximately 17.45m.

Wearing glasses: Many binoculars provide the full field of view when wearing glasses by either turning/pushing the retractable eyecups to the ‘down’ position or folding down the rubber eyecups. Binoculars with a stated eyerelief of 15mm or more deliver the full field of view for most spectacle wearers.

Weight For most people, small, lightweight binoculars will be used much more than large heavy ones which tend to be left at home or in the car.

Wikipedia has a good guide too: https://www.wikihow.com/Choose-Binoculars