Binoculars

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A typical pair of general-use binoculars

Binoculars are an optical aid used to view distant objects with both eyes, such as scenery, wildlife, or far off boats or ships. They can also be employed in basic astronomy, in lieu of a telescope.

They consist of twin barrels containing a system of lenses one for each eye. Because the distance between the eyes varies from person to person, there must be provision for adjusting the distance between the binoculars' eyepieces; thus the barrels are hinged together.

While most binoculars are simply designed to be purely functional, opera glasses are a kind of binoculars that are usually rather ornate in finish. They are used to improve the patron's view of the action on stage. The fancy form fits in with the formal nature of a performance and the highly decorated interior of most theaters.

Contents

Settings

Binoculars usually have three settings. One of them is used to change focus the binoculars on objects of different distances. The other two are personal settings which vary from person to person but which, for any person, only need to be set once and do not need to be changed subsequently.

1) A focusing knob is usually provided in the center which focuses both sides at the same time, to focus the binoculars on objects of varying distances.

2) The distance between the eyes (the intraocular distance) varies from person to person. In order to allow the distance between the binocular tubes to be changed, they are hinged together at the center. Often there is a graduated scale at the hinge from which the intraocular distance can be read, usually in millimeters. If a user remembers their intraocular distance, they can quickly bring any pair of binoculars back to setting that is proper for them.

3) A second focusing adjustment is provided, usually on the right eyepiece, usually with a scale and markings in diopters. For many people, this adjustment should be set to the zero mark. Some people will find that because of refractive differences in their eyes, when the left binocular is perfectly focussed, the right binocular will be slightly out of focus. It can be brought into focus by adjusting the right eyepiece focus. Once the right eyepiece focus is set correctly, it will be correct at all distances and does not need to be adjusted again.

Designs

Because they are used to view terrestrial objects, the images in a pair of binoculars need to be erect, i.e. right-side-up. This poses an engineering challenge. There is a kind of telescope, the Galilean telescope, which uses only lenses and provides an erect image. Some instruments use a pair of Galilean telescopes. This design is inexpensive, can be compact, and can produce a sharp image; however, their magnification, field of view, and "exit pupil size" (which determines brightness) are all limited. This design was once popular for opera glasses, but today is found mostly in children's toy binoculars.

Most modern binoculars are prismatic binoculars. The lens systems used in them are similar to the design of a "astronomical" refracting telescope, but a system of erecting prisms is added to turn the image right side up.

In the traditional design, the presence of a prism system is visible in a dogleg appearance of the two barrels, the eyepieces being offset and out of line with the main barrel. However, in recent decades, optics firms have developed very compact prismatic binoculars with prism systems are inline with the barrels with no visible offset.

Exit pupil size

Binocular designs trade off between cost, size, and optical characteristics. One very important characteristic is the exit pupil size, which can be calculated by dividing the objective diameter by the magnification, an affects what uses the binoculars are good for.

  • In a pair of "7x50" binoculars, which might be called "night glasses," the exit pupil is 50 mm / 7X magnification = 7.1 mm
  • In a pair of "7x35" binoculars, which is a general-purpose specification, the exit pupil is 35 mm / 7X magnification = 5 mm
  • In a pair of "8x20" compact binoculars, the exit pupil is 20 mm / 8X magnification is 2.5 mm

The exit pupil affects image brightness in that[1]

  • If the exit pupil is the smaller than the pupil of the eye, the image seen through the binoculars will be dimmer than the image seen with the unaided eye.
  • If the exit pupil is the same or larger than pupil of the eye, the image seen through the binoculars will be just as bright as the image seen with the eye (or perhaps very slightly dimmer due to light loss in the lenses and prisms).
  • The image is never brighter than the image that can be seen with the unaided eye.

The pupil of the eye varies in size depending on how bright the light is. At night, the dark-adapted eye expands to about 7 mm. In room light or shady daylight, it is about 3-4 mm. In very bright daylight it may be less than 2 mm.

What this means is that, in the examples above, night, the 7x50 "night glasses" will give a much brighter image than the other two. Under ordinary conditions, the 7x50 will not be any brighter than the 7x35s (and are considerably heavier and more expensive). And in bright daylight, there may not be any meaningful differences in brightness between the three (and if the 8x20s are slightly dimmer, that may be a good trade-off in return for their compactness and light weight).

Warning!

You must on no account point binoculars at the sun. If you look at the sun directly without any eye protection through binoculars you will be blinded. On a sunny day take care that the binoculars point well away from the sun.

References

  1. When looking at ordinary "extended objects," e.g. birds, targets, distant ships. The rules are different for astronomical point objects like stars
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