Binoculars are basically two telescopes attached together and aligned exactly enabling the observer to use both eyes when viewing. As such binoculars are able to provide quite good depth of field (a stereo vision effect).
In addition, a binocular has a set of prisms fixed into the body that inverts the light path, so that the image you see through a binocular is as you see it with your naked eye.

Two main variations of binoculars exist :

• Roof Prism &
• Porro Prism

Both types have their benefits although for the casual user, either is usable. The two types stem from how the prisms are placed within the binocular assembly. These prisms invert the image the right way up and around.

All binoculars are marked as magnification x aperture (mm) eg 7x50, 10x56, 8x22 etc.
Magnification is the amount by which the binocular brings an object closer. For instance at 10x an object at 100m distance would be viewed as if it were only 10m distant.
The aperture is the diameter (in mm) of the front lens of the binocular. Generally the larger the aperture, the more light will pass through the binoculars and reach your eye.

Another important figure to note is the 'exit pupil'. This is the ratio of the magnification to the aperture of the binocular. The exit pupil defines the diamater (mm) of the illuminated circle that can be seen when holding the binocular at arm's length and looking through one of the eyepieces.

Binoculars that have an exit pupil of above 5 mm are suitable for night-time astronomical use. This is due to the fact that the pupil of the eye expands out to 5-7mm during the night.
Aperture is an important consideration when looking for astronomical binoculars. Generally the minium aperture to look for is 50mm.

An important item to consider is that all binoculars are suitable for daytime use, although different situations will require a different style of binocular. For instance for theatre a compact binocular would be best or for sports a robust but light model may be suitable.

Below is a table outlining three different binoculars and their uses :

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