Magnification is represented by a number and indicates how large an object appears when viewed through the binoculars. With a magnification factor of 8x, for example, a subject 800 m away appears as if it were 100m away. Magnification factors from 8x to 10x allow for easy viewing, outdoors and indoors, whereas higher magnification requires an absolutely steady hand or some support for comfortable viewing.

The diameter of the objective lens – in combination with the quality of prism and lens coatings – determines how much light can be gathered to form an image. For daytime use, a lens diameter of 21–25 mm is recommended, but for low light conditions, 35-50 mm provides better performance.

The real field of view is the angle of the viewing field measured in degrees and indicates how large the viewable field is at a distance of 1000 m from the point of observation. Sometimes the field of view is also expressed as width (in metres).

The exit pupil is the diameter of the light beam visible through the eyepiece. It is calculated by dividing the objective lens diameter by the magnification factor. The larger the exit pupil diameter, the brighter the image seen. An exit pupil of 2–3 mm is adequate for daytime use, while in low-light conditions an exit pupil of 5–7 mm is preferable. For example, with 8×42 binoculars: 42 divided by 8 = 5.3 mm.

Brightness is a relative value, obtained by squaring the diameter of the exit pupil. Larger relative brightness values generally produce brighter images. However, this is not always the case, as the amount of light the eye can absorb depends on the actual diameter of the pupil, which varies between bright and dark surroundings.

The model names provide an overview of the most important features. For example, the description 10×42 EXPS I means that the model offers 10× magnification and an objective lens diameter of 42 mm. To calculate the exit pupil, divide 42 by 10 = 4.2 mm.

Eye relief is the distance from the rear eyepiece lens to the eye point. Longer eye relief provides comfortable viewing and ease of use without image blackout. Insufficient eye relief results in a loss of field of view. For users who wear glasses, an eye relief with at least 14 mm is recommended.

Porro prism (left): Large lenses produce needle-sharp, bright images with excellent quality and a distinct sense of depth. Roof prism (right): The light is aligned directly with the eyepiece, allowing these binoculars to be made lighter and more compact.

The shortest distance at which binoculars can be focused is called the minimum focusing distance. When observing close-up objects such as flowers or insects, binoculars with a short minimum focusing distance — such as the 8×42 PRO or 10×42 PRO, which have a minimum focusing distance of 1.5 m — are more convenient to use.