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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.