Binocular essentials, useful information about binocular terminology
The binocular formula (10x50 etc.)
The numbers you see on a binocular are quite important and useful, and tell you much of what you need to know about any binocular (or spotting scope). Fortunately, and unusually, a convention has been adopted worldwide for the technical specification of binoculars. The first number of the formula (as in 10x50) is always the power or magnification. This means the object viewed will be ten times larger (or closer if you prefer) than you would see it with the unaided eye. If the magnification is variable, as with a zoom binocular, it will usually be written as 10-24x50 with the dash indicating that that the power can be varied anywhere between 10 to 24 times what would be seen with the unaided eye. When offering zoom binoculars on our website, for the sake of clarity we have normally used the word ‘to’ instead of the dash.
The second number in the formula (as in 10x50) always refers to the object lens (the ones at the front) diameter in millimetres. So in the case of 10x50 binoculars the primary or objective lenses have a diameter of 50mm (about 2 inches). The larger the objective lens, the more light will enter the binocular. (Pretty obvious really, a big hole in your boat lets in more water than a small one!). Big lenses offer more than just image brightness — larger lenses offer gains in image sharpness and exit pupil (see Exit pupil below) size as well.
Prism types
These are the essential wedge-shaped blocks of glass inside the binocular that are used to fold the long focal length of the primary optics (the lenses at the front of the bino) into something more manageable, so that the binoculars are easy to use and carry rather than being 3 or 4 foot long! There are two types of prisms used in binoculars:
Porro prisms — Historically, all binoculars were porro prism design. The larger porro prism binoculars are easily recognised by the distinctive dog-leg shape — with the objective lenses spaced much wider apart than the eyepieces. The wider-spaced objectives of porro binos offer very slightly better stereoscopic depth perception than same-spec slimmer roof prism versions. Generally, porro prism binos offer a slightly wider field of view than comparable roof prism types. Pound-for-pound, porro binoculars tend to be slightly better value than roof prism. This is not easy to quantify exactly, but in general terms the porro design is significantly easier to manufacture and this tends to lead to the observation that you have to spend a little more on roof prism binos of the same general specification to get the same performance.
Roof prisms — A relatively recent innovation compared to the porro type, the prisms here overlap more closely, allowing the objective lenses to line up directly with the eyepieces and thus creating a slim, sometimes partly foldable, streamlined shape where all the lenses and prisms that magnify and correct the image are mounted in an optically efficient straight line. The main benefit of the roof prism design is the reduction in physical size compared to same spec porro types. Many users also report that the roof prism types are more comfortable and convenient to use.
Exit pupil
This is an important measure of binocular specification. The exit pupil of a binocular is described in millimeters and is the diameter of the narrow beam of light that leaves the eyepiece and enters the user’s eye. The larger the exit pupil, the brighter the image obtained will be. Having a large exit pupil tends to be more important when viewing under low light conditions and at night. But a larger exit pupil usually can make the binocular easier and more comfortable to use and minimises the effect of vibration and shake.
To calculate the exit pupil, divide the size of the objective lens by the magnification of the binocular. For example, the exit pupil of a 10x50 binocular is 50 ÷ 10 = 5mm. A wide range of decent exit pupil sizes is acceptable but beware of binoculars with tiny exit pupil values. Binoculars that boast very high magnifications with small objective lenses are quite useless (we’ve seen 80x25 offered for sale! They’ll have an exit pupil of 0.31mm and all the hapless user will see is a very dirty dim image and swarms of ‘floaters’. The reason the floaters show up is that with such a narrow exit pupil the normal cellular debris that floats in the fluids of the eyeball will cast shadows on to the retina and make the binocular, at best, unsatisfactory and at worst positively unpleasant to use!
Eye relief
This is another useful and important feature of binocular performance. The term eye relief refers to the distance that a binocular can be held from the eye and allow the full field of view to be comfortably observed. Eyeglass wearers in particular benefit from longer eye relief.