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The second of two figures quoted on a pair of binoculars that indicates the diameter of the lens furthest away from your eye when you're looking through them. It's quoted in millimeters and the first figure is the magnification. A 10x50 pair, for example, has 10x magnification with a 50mm objective lens.
The circle of light that you see when you look through a pair of binoculars the wrong way round (when held at arms length). The exact size can be measured by dividing the objective lens by the magnification of the binoculars. A 10x50, for example, would have an exit pupil of 5. This figure, also referred to as the brightness index, is most important in low light. As a guide you should ensure the binoculars have a similar sized exit pupil to our own eyes' pupils. In low light our eyes' pupils open to about 5 to 7mm wide to allow more light to pass to the retina. So if you intend to use the binoculars at night, indoors or in dense woodland, choose a pair with a similar value exit pupil. In bright conditions the pupils contract to about 2 or 3 millimetres and in such conditions the extra transmitted light isn't needed.
Indicates how good binoculars are in low light. To find this divide the objective lens by the magnification and square the result. A 10x40 has a relative brightness of 16 (40/10= 4, 4x4=16). A higher number means the binoculars will be better in low light situations.
An indication of the brightness value of binoculars. It's similar to relative brightness, but you multiply the objective lens with the magnification and then find the square root of the resulting number. A higher number indicates the binoculars will be better for use in low light.