The tubes have lenses that focus light and magnify the image, a process called magnification. This way, when someone looks through the binoculars at a far-away object, it looks much closer. They also have prisms, which bend the light so to shorten the length of the tubes. That is why a binocular is shorter than a telescope. A binocular image is produced by two short telescope tubes.
Binoculars were invented in the 17th century. They do the work of a telescope in a more compact form: they take up less space. The Royal Navy used telescopes, but now all ships have heavy binoculars. They can be mounted on swivels to scan the horizon. Binoculars are also standard issue in all armies. Night vision binoculars use photocathode electronics to intensify the image.
These features are present in some or all binoculars:
- Focussing means, usually knobs you turn.
- Independent focussing for each 'telescope' (each side) is used on all large binoculars.
- Eye relief: Eye relief is the distance from the rear eyepiece lens to the exit pupil or eye point. It is the distance the observer must position his or her eye behind the eyepiece in order to see the image. The longer the focal length of the eyepiece, the greater the eye relief. Binoculars may have eye relief ranging from a few millimeters to 2.5 centimeters or more. Eye relief can be particularly important for eyeglass wearers.
- Range finding. Many binoculars have range finding reticle (scale) superimposed on the view. This scale allows the distance to the object to be estimated if the object height is known.
- Image stabilisation. On top-range models, by electronics.
- Optical coatings
- Anti-reflective: not just relections off the front lens, but every lens and prism. Anti-reflective coatings reduce light lost at every optical surface through reflection at each surface. Reducing reflection via anti-reflective coatings also reduces the amount of "lost" light bouncing around inside the binocular which can making the image appear hazy (low contrast).
- Metallic mirror coating for its prisms.
- Very large binocular naval rangefinders (up to 15 metres separation of the two objective lenses, weight 10 tons, for ranging World War II naval gun targets 25 km away) have been used, although late-20th century technology made this application redundant (radar; laser sights; guided missiles &c).
- Modern military binoculars have filters that block lasar weapons used against them.