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The basics of binoculars

You have to understand the basics of binoculars before buying one. This is the purpose of this article.

A binocular is actually two perfectly aligned telescopes  and getting  these two telescopes to work together as one instrument is a huge challenge for manufacturers. The physical features of binoculars which strike you first, are basic design, central focusing mechanism and diopter.

Basic design: There are two basic and conspicuously different types of designs. All binoculars either use “porro prisms” or “roof prisms”. The ones using porro prisms are the traditional design where the eye pieces are off-center to the barrels, whereas the ones using roof prisms the eye pieces are in line with the objective lenses and have a sleek design. Porro Prism binoculars are less expensive and have greater three-dimensional viewing effect, whereas roof prism binoculars are more compact and lighter, but are usually more expensive.

Central focusing mechanism: The knob/wheel between the two optical barrels by means of which you change the focus when looking at an object. It could also be a toggle switch.  The focus of “Auto focus” binoculars have been fixed at the factory, so they do not have a central focusing mechanism.

Diopter: This is a focus knob (usually at the right eyepiece) that lets you focus one side of the binoculars separately from the other, in order to set the binoculars to accommodate differences in the focusing of your two eyes. Some “auto focus” binoculars have a diopter for each eyepiece.

Technical features:

Magnification/power: A set of technical detail imprinted on any pair of binoculars could be something like this: 8×42, or 10×50. The first number (8, 10) is the magnification number, the “power” of the binoculars. The object is magnified by 8 or 10 times, which means that it appears to be 8 or 10 times closer than seen with the naked eye.  Please note: The more powerful a binocular, the less bright the image – which will be a problem when focusing on something in the shade of a tree or in dim light in general. Moreover, the more powerful, the more difficult it becomes to focus on an object: Even the slightest hand movement is enhanced. Some binoculars do have image stabilization which electronically reduces movement for steadier viewing, but these are very expensive. A magnification of 8 xs is generally regarded as ideal for general, game and bird viewing.

Objective Lens: The second number (42, 50) is the diameter of the objective lens (in millimeters), i.e. the two big lenses at the front of the two barrels.  It is also called the “aperture” of the binocular. The bigger the aperture, the more light is captured, resulting in a clearer and brighter image, which is important when used in fading light.

Field of view (FOV): Another set of technical detail which is imprinted on the binoculars, could be something like this: 400 ft. at 1000 yds. This is the field of view, the width of the view at the particular distance. In this case 400 feet at 1000 yards. Could be designated in meters or in degree. Field-of-view is determined eyepiece design, but magnification does have an effect as well. For this reason even small Pocket Binoculars can have a reasonable FOV.

The exit pupil: The exit pupil can be seen by holding the binoculars at arm’s length and looking through the eyepieces. The pencil of light you see is the exit pupil. The actual diameter of the exit pupil is computed by dividing the diameter of the front objective lens (in millimeters) by the magnification of the binocular. So, in the case of a 7×35 binoculars, the diameter of the exit pupil is 5 millimeters. The relative brightness index (RBI) is an indication of image brightness. It is computed by squaring the exit pupil. So the RBI of our example is 25 (5×5=25). A RBI of 25 or greater is considered good for use in dim light.

Eye relief: For every pair of binoculars, there is an optimal distance between your eyes and the eyepieces, which is called eye relief. If your eyes are too close to or too far from the eyepieces, you can’t see the whole picture: part of it is blacked out. So, if you happen to wear eyeglasses, you have to make sure the eye relief is at least 15 mm. Except in the case of very expensive binoculars, field of view and eye relief work against one another. If the manufacturer increases the one, it is at the cost of the other.

Coating of lenses:The lenses of all binoculars should be made from Bk-4 glass and are coated with anti-reflective coating, which is applied to enhance the quality of the image. Make sure you choose a “Fully Multi-coated” pair of binoculars.  In the case of roof prism binoculars the lenses also have to be coated with “phase correcting” coatings.

Waterproofing: A waterproof model will not only be less likely to fog up internally, it will also be better sealed against dust and dirt.  Make sure the binoculars are “nitrogen purged/filled”.

Close focus: Focusing at a distance is no problem for any pair of binoculars; focusing at close range, however, is a totally different matter. For watching birds or butterflies you will have to look for a pair with a close focus range of less than 4 meters.  Compact Binoculars excel in close focus, some even as close as 1 meter.

The focus speed: If you need the binoculars for watching wild animals, in particular birds, the speed of focus is important. While following a bird in flight, you need to change the focus all the time and if your binoculars do not allow for quick adjustment, you will struggle all the way.

About the Author

Cobus Cronje has written extensively on a wide variety of subjects, including binoculars. You can get more information on pocket binoculars.

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