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Terminology used in Binocular articles

Figuring out what the terms used to describe binoculars is the hardest part of doing research on binoculars. Binocular jargon can be confusing. Here is the list I have compiled.

The terms used in binocular articles are very similar and overlap with other optical instruments such as telescopes and cameras, no surprise here.

Angle of View
The angle which is visible through the binoculars given in degrees. How wide an area can be seen. Wider angle of view is useful if you have to follow something in motion such as in sports meets or in birdwatching. SEE ALSO: field of view which is the same as angle of view but which reports the viewed diameter in feet per 1000 yards.
Apparent Angle of View
This is the value of the field of view multiplied by the magnification (Apparent field of view = Magnification x Real field of view). Using apparent angle of View allows you to compare binoculars of different magnification.
Broadband coatings
Most Lens coatings are more effective at reducing reflections in the middle range of visible light. Broadband coatings have been developed and are better at reducing reflections through the whole range of the visible spectrum.
Chromatic Aberrations
When white light passes through glass lenses not all colours are bent at the same angle and this results in colour fringes. This is because not all colours have come in focus at the same point. Lens are combined into lens elements to eliminate this. The greater the magnification the more serious the distortion can be. ED glass has been developed and is less subject to chromatic abheration
Close Focusing Distance
Is the closest distance the binocular will be able to focus. Marine binoculars will tend to have longer distances while binoculars used for birdwatching will focus at a closer distance.
The lining up of the various optical elements in binoculars. If you look through a pair of binoculars and see a doubling of the image rather than one sharp image well focused, it means that the optical elements are out of collimation. This is caused by rough handling or knocks, just normal wear and by improper alignment at the factory. Larger binoculars are more likely to get out of alignment. Your eyes are good at compensating for minor mis alignments but it is possible to align good binoculars by accessing adjustment screws inside the body.
ED Glass
ED (Extra-low Dispersion) glass is used in lens elements to greatly reduce chromatic aberration. It does not bend light as much as regular glass and so the rainbow effect and colour fringing is much reduced.
Eye Relief
The distance between the occular lens and the eye that allows the whole field of view to be visible. Longer eye relief allow viewers to keep the binocular at a greater distance from the face, it also allows user to wear glasses if necessary and is more comfortable. It also reduces condensation on the occular lens from moisture from your eye, if you are using the binoculars in cold weather. The term exit pupil distance is sometimes used.
Exit Pupil
If you point binoculars at a bright light and look through the occular from a distance (a foot) you will see a bright circle, that's the exit pupil. Diameter will vary depending on the size of the objective and the eyepiece. In a binocular of 7x35 That is a magnification of 7 and an eyepiece of 35 mm, the size of the exit pupil will be 35 / 7 =5 mm.
For best viewing an exit pupil of a diameter approximating the eye's pupil diameter is best. If the exit pupil of the binocular is larger than the eye's pupil, light will be lost instead of entering the eye, this is not important if there is lots of light but if the viewing is in low light then the image suffers by being dark; if the exit pupil is smaller than the pupil opening the view will be vignetted. Human pupil change in size depending on the light conditions. Small opening when bright, larger when dark. If you are planning to look at stars or birds in dark forest then you don't want to lose light. If you are looking at bright boats or football games in sunny conditions then losing light is not a problem. A 7×50 binoculars has an exit pupil just over 7 mm, which is close to a young person's dark adapted eye pupil.The light from the binocular at the eyepiece fills the eye's pupil, meaning no loss of brightness at night. In daylight, when the eye's pupil is reduced to 4 mm in diameter, over half the light will be blocked by the iris. BUT, the loss of light in the daytime is generally not significant because there is lots of light already. Similarly 8×32 binoculars, have an exit pupil of only 4 mm. (32/8=4mm) That is enough to fill a typical daytime eye pupil, but not for nightime sized pupil. The pupil size of a human eye is between 5–9 mm for people 25 or less, This decreases slowly with age.
Field of View
numbers on binocularsHow wide an area you can see. It is given 2 different ways. Some binoculars give a field of view in feet at 1000 yards. This is the diameter of the area you see at a distance of 1,000 yards. Other manufacturers give an angle of view in degrees. This is the slice of a circle you can see. If it were possible to see all around at once you would have an angle of view of 360 degrees. If you multiply the angle of view by 52.5 you get feet at 1000 yards. If you divide the number of feet by 52.5 you get the angle of view. The term angle of view and field of view seem to be used interchangably but strictly speaking the angle of view is in degrees and the field of view is in feet. SEE ALSO Apparent Field of View.
Fog Proof
Some sealed binoculars are filled with an inert gas which has been dried. This prevents humidity from getting inside the binoculars and so there is no condensation possible on the inside of the lens and prisms when the temperature drops.
Interpupillary distance IPD
The distance between the centre of the pupils of the eye. It varies and most binoculars are hinged in the centre to allow you to adjust the binoculars so the pupils are properly aligned to the occular lens.
Image Stabilized Binoculars ISB
Several companies offer stabilization as a feature. It reduces the image shake as your arm or the boat moves. It allows for hand holding a larger objective binocular. Image stabilization uses battery powered sensors and adjustors that modify the image path as it moves. Cannon has several models. Image stabilization is a feature of higher end binoculars.
Porro Prism
They bend the optical path in a Z pattern. Because they are bulkier, binoculars with porro prisms tend to be the wider traditional shapes. Because of the angle of the optical path porro prisms do not lose light, they have better reflective quality. Binoculars made with porro prisms are easier and less expensive to make. They are also larger and often heavier. Because binoculars made with Porro prisms permit the barrels of the binocular to be further apart, it allows for greater depth of the image 3D
Are used to modify the path of the image through the binoculars (optical path). They allow binoculars that are wider than the distance between your eyes. There are simple small binoculars that do not have prisms called newtonian binoculars, but most do. Wider distances give better 3-d images.There are different prism materials used. You will hear of BK7 which is Borosilicate Glass. A current favourite is BAK4 which is Barium Crown Glass. It is said to transmit light better. There are 2 types of prisms in common use. Porro Prisms and Roof prisms.
Relative Brightness
This measures how bright an objects appears to your eyes. Higher number means brighter conditions. Relative brightness is the exit pupil squared.
Roof or Dach prism
Because of the geometry of the roof prism (named because they look like little roofs) the first element does not reflect light perfectly and unless the elements are very carefully made and coated, there is a danger of loss of light and light flashes. Manufacture is more expensive. The benefit of the roof prism is that it allows for a more compact optical path and the resulting binoculars can be narrower and smaller and lighter.
Lens Coating
In order to allow as much light as possible to be transmitted through the binoculars, the various elements are coated with finishes that reduce reflection. Coatings can reduce reflection from 5% or more to less than .25%.

There are 4 levels of coating: Coated refers to a single layer on at least one lens, usually the objective. Fully Coated means that all air to glass surfaces are coated, multi coated means that there are multiple layers on at least one lens and fully multi coated means multiple layers on all air to glass surfaces. Some manufacturers bend the truth about their coatings though particularly at the lower end of the price range.

Lens Elements
In good optical instruments, the objective and occular are made up of several lens glued together. This is done to avoid distortions such as chromatic aberrations and improve sharpness.
Magnification or Power
Shown as the first number on the binoculars, magnification will range between 4 and 10 for the most common instruments. More specialized binoculars will magnify much more but are more difficult to handle and keep steady. The greater the magnification, the narrower the field of view.
Ruby Coating
A proprietary coating which was developed by Steiner for use in hunting and nature binoculars. It was said to improve the detail and contrast in browns and eath colours. The ruby (contains no ruby) bandwagon was adopted by poorly made binoculars to hide the chromatic aberration of their lens. Since it filters out some of the reds the colour fringe is reduced. Ruby coated lens tend to produce an image that is lacking in colour saturation.

email me if you find mistakes, I'll fix them and we'll all benefit: Christine