There are a lot of special terms when it’s about binoculars and telescopes. Find the most common terminology in this brief binoculars glossary:
Eyecups: They ensure the correct distance to the eyepiece. Spectacle wearers correct simple visual defects with the binoculars. The glasses should only be used if the compensation on the glass does not work.
Exit Pupil: is the diameter of the light beam which leaves the binoculars or telescope at the ocular (eyepiece). The exit pupil is the quotient of aperture diameter by the magnification.
Eye Relief: The distance between the ocular lens and your eye’s pupil is called eye relief. At this distance, the maximum field of view can be comfortably observed.
Diopter adjustment: Corrects the different visual strength of both eyes. Spectacle wearers should try to balance the different eyesight with the binoculars. Use the diopter adjustment as follows:
-Use the left eye to aim at the target (right eye closed) and turn the central focusing wheel until in focus.
-Now use the right eye (left eye closed) and turn the diopter adjustment on the right ocular tube until the image is focused. Done, now only the center focus is used – until another one uses the scope.
Field of view: This is the horizontal width of the image you see when looking through the binoculars. Usually, this is a value in feet (meters) of the width of the area at a distance of 1,000 yards (or 1,000 m). In simple terms: the smaller the field of view = the larger the magnification.
Focus Wheel: The central wheel to adjust a blurry image to become focused. Turning the central focus wheel slightly changes the distance between the ocular lens and the objective lens until the image is in Focus.
Gas filling: High-quality glasses are filled with Nitrogen or Argon gas. The absence of normal air and humidity inside the instrument prevents condensation and fogging, thus allowing use in extreme temperature fluctuations and reducing the risk of lens fungus.
Lens Distance: The further the objective lenses are apart, the more “three-dimensional” becomes the viewing experience.
Magnification: The magnification (or power) refers to the degree at which the image you are viewing is enlarged. For example, a 10×42 binocular magnifies the image 10 times or in other words, an object that is 1000 yards away, appears only to be 100 yards away.
Opening: This is the diameter of the objective lens. Specs 8×42 refers to an objective of 42 mm diameter. The larger the aperture the more light is collected and the scope becomes more suitability for uses in low light conditions. Night glasses or stargazing binoculars usually have apertures of 50 mm upwards.
Prisms: Prisms made of high-quality glass are located inside the tube of the binoculars for two purposes. They correct an inverted image so the world is not upside down. They also increase the focal length while reducing the overall length of the instrument. (Longer focal length will allow for higher magnification). There are two types of prisms used in binoculars, Porro prisms, and Roof prisms. Porro Prisms give binoculars the typical two-stage profile, while Roof Prisms produce a straight optical channel.
Twilight performance: How good binoculars perform in twilight depends on several factors: light transmittance (values over 90 percent are possible), the size of the aperture (the more, the better), lens coating and the size of the exit pupil.
Weight: Heavy glasses are more steady in the observer’s hand and allow for concentrated observation of details. However, they may be tiring after a while.