As a general rule, the higher the magnification, the narrower the field of view, the larger the image appears, and the duller the image becomes in low light. High magnification binoculars vie a reduced depth of field, and the higher the magnification, the more natural handshake affects the image. A support such as a tripod may be used when viewing through a high magnification binocular for any length of time.

For normal hand-held use, magnification of 8x or 10x is recommended.

Say for example you have an 8 x 40 binocular. The first figure refers to the magnifying power, so this binocular magnifies by 8x, making objects seem eight times nearer to you than they actually are. The second figure refers to the width of the objective lens measured in mm. The wider the lens, the more light is admitted to the binocular, so this 8 x 40 model will admit twice as much light as an 8 x 20 model.

The amount of light which actually reaches the eye is known as the exit pupil diameter (EPD) and is measured in mm. It can be found by dividing the lens diameter by the magnification - in this example, 40 divided by 8 means this binocular has an EPD of 5mm.

For regular use at a particular time of day, a binocular with a higher EPD is preferred for use at dawn or twilight, as it admits more light, and a lower EPD for bright sunlight.

For general use, an EPD of around 5mm (eg 8 x 40 binocular) is recommended.

The prisms inside a binocular are triangular blocks of glass used to reflect light. They are used in order to reduce the length of the binocular, otherwise it would be the length of a telescope. A Porro prism system is used in a conventional shaped binocular, and provides better 3D perception. A roof prism system is more compact and consequently these binoculars are smaller. A silver-coated roof prism is more reflective, so less light is lost within the optics system, providing a brighter image. Most optical prisms are made from BK-7 (borosilicate) glass or BAK-4 glass. BAK-4 is higher quality and results in brighter images with sharper edges.

This is the distance which is in focus at any one time. Generally, the greater the magnification, the shorter the depth of field.

The field of view refers to how much you can see through the binocular, usually expressed as the width of panoramic view when seen from a distance of 1000m. A narrower field of view is generally preferred by people wishing to look at small objects such as birds. A wider field of view is more suitable for looking at the wider landscape.

Many binoculars can be used by spectacle wearers. They either have rubber eyecups which fold down, or eyecup assemblies which can be adjusted.

On auto-focus binoculars, the focus is pre-set at infinity so no adjustment is necessary for different distances. Most auto-focus binoculars will be in focus from about 20m to infinity. Manual focus binoculars are needed for observation at shorter distances, and the minimum focus distance varies with different models.

Binoculars vary considerably in size and weight. Small, lightweight binoculars which can be taken anywhere can be an advantage when travelling or outdoors.

The large magnifying lens at the far end of a binocular. Lenses are coated to reduce light loss and glare. There are four degrees of coating quality:

Coated: Single layer on at least one lens surface
Fully Coated: Single layer on all air-to-glass surfaces
Multi-coated: Multiple layers on at least one surface
Fully Multi-coated: Multiple layers on all air-to-glass surfaces

Most binoculars have rubber armouring which protects them from everyday bangs and scrapes. It can either be smooth (easier cleaning) or rugged (better grip). Some are fully waterproof - ideal for watersports - and have been purged with nitrogen to prevent them fogging up whatever the weather.