Binoculars or field glasses are two telescopes placed side by side and aligned to point in the same direction, enabling the spectator to use both eyes (binocular vision) when observing distant objects. Many are of the scale to be carried with both hands, although the proportions range considerably from opera glasses to large pedestal-mounted military versions.
Binoculars are typically designed for special uses. These various designs require some optical specifications which may be shown on the binocular cover plate of the prism.
As the first number in the binocular definition, the magnification is the measure of the focal length of the target separated by the focal length of the eyepiece. This brings the magnifying power to the binoculars. For eg, a magnification factor of 7, creates an image 7 times greater than the original seen from that point. The desired level of magnification depends on the intended use and is a permanent, non-adjustable feature of the system in most binoculars. Hand-held binoculars usually have magnifications ranging from 7x to 10x, making them less sensitive to the effects of trembling hands. A greater magnification leads to a narrower field of view which can require a tripod to preserve image stabilization. Any adapted binoculars for astronomy or military use have magnifications from 15x to 25x.
Provided the second number in the binocular definition, the diameter of the objective lens determines the resolution and how much light can be collected to create the image. When two separate binoculars have the same magnification, the same accuracy and produce a sufficiently matched exit pupil, the greater objective diameter produces a brighter and clearer image.
Field of view:
The field of view of the pair of binoculars depends on its optical design and is, in general, inversely proportional to the magnifying power. It is generally noted in a linear value, such as how many feet of width can be seen at 1,000 yards, or in an angular value of how many degrees can be seen.
The binoculars focus the light captured by the lens into a beam whose diameter, the outlet pupil, is the target diameter separated by the magnifying force. If the cone of light streaming out of the binoculars is greater than the pupil it is entering, any light larger than the pupil is being lost.
This ease of positioning helps to prevent, particularly in wide field of view binoculars, vignetting, which gives to the viewer an image with borders obscured so the light from them is partially blurred, and it ensures that the image can be easily found which is vital when looking at birds or game animals that move rapidly, or for a seafarer on the deck of a pitching boat or submarine.
Relieving the eye is the distance from the rear eyepiece lens to the outlet pupil or point of the eye. It is the distance from which the viewer must put his or her eye behind the eyepiece in order to see an unlined image. The larger the focal length of the eyepiece, the higher the opportunity for relaxation of the eye. Binoculars may have an eye relief of between a few millimetres and 2.5 centimetres or more. Eye relief may be of special significance to eyeglass wearers. The eye of the eyeglass holder is usually further away from the eye piece, which needs a longer eye relief in order to prevent vignetting and, in serious situations, to maintain the full field of view. Binoculars with short eye relief can also be difficult to use in situations where it is difficult to keep them straight.
Since the standard binocular has 6 to 10 optical components with unique properties and up to 16 air-to-glass surfaces, binocular manufactures use various forms of optical coatings for technological purposes and to enhance the image they create.
Anti-reflective coatings minimise the loss of light on each optical surface by focusing on each surface. Reducing reflection by anti-reflective coatings often decreases the amount of wasted light within the binocular that would otherwise make the image look hazy. A pair of binoculars with good optical coatings can create a brighter image than uncoated binoculars with a larger objective lens due to the superior light transmission through the assembly.
Phase correction coatings:
In the binoculars with the roof prisms, the light stream is separated into two directions reflecting on each side of the roof prism ridge. Binoculars with either a Schmidt–Pechan roof prism or an Abbe–Koenig roof prism benefit from phase coatings. Porro prism binoculars do not separate the beams and thus do not need any phase coatings.
Metallic mirror coatings:
In binoculars with Schmidt–Pechan roof prisms, mirror coatings are applied to certain surfaces of the roof prism so the illumination appears at one of the glass-air border of the prism at an angle less than the critical angle such that absolute internal reflection does not occur. Without a mirror covering, much of the light will be lost. Schmidt–Pechan roof prism aluminium mirror coating or silver mirror coating is used.
Dielectric mirror coatings:
Dielectric coatings are used to allow prism surfaces to behave as dielectric mirrors. The non-metallic dielectric reflective coating consists of multiple multilayers of alternating high and low refractive index materials deposited on reflective prism surfaces. This multi-layer coating increases the reflectivity of prism surfaces by acting as a distributed Bragg reflector.
Bird Watching is a very common activity amongst nature and animal lovers; binoculars are their most essential instrument since most human eyes cannot solve enough detail to completely understand and/or study small birds. Usually binoculars with magnification from 8x to 10x are used, but several manufacturers offer 7x magnification versions for a wider field of vision.
Hunters commonly use field binoculars as a way to see game species that are too far away to spot with the naked eye. Hunters more often use 8x light relay binoculars and large enough lenses to capture light in low light conditions.
Considering the binoculars are intended to add insight to your outdoor activities, it’s surprising how complicated things get as you want to determine which pair of binoculars is right for you. The easiest way to test the optical efficiency and ease of use for a pair of binoculars is to go to a shop and browse at some of them before you make a final decision.