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Binocular designs trade off between cost, size, and optical characteristics. One very important characteristic is the ''exit pupil size,'' which can be calculated by ''dividing'' the objective diameter by the magnification, an affects what uses the binoculars are good for.  

Binocular designs trade off between cost, size, and optical characteristics. One very important characteristic is the ''exit pupil size,'' which can be calculated by ''dividing'' the objective diameter by the magnification, an affects what uses the binoculars are good for.  

*In a pair of "7x50" binoculars, which might be called "night glasses," the exit pupil is 50 mm / 7X magnification = 7.1 mm

*In a pair of "7x50" binoculars, which might be called "night glasses," the exit pupil is 50 mm / 7X magnification = 7.1 mm

*In a pair of "7x35" binoculars, which is a general-purpose specification, the exit pupil is 35 mm / 7X magnification = 5 mm

*In a pair of "7x35" binoculars, which is a general-purpose specification, the exit pupil is 35 mm / 7X magnification = 5 mm

*In a pair of "8x20" compact binoculars, the exit pupil is 20 mm / 8X magnification is 2.5 mm

*In a pair of "8x20" compact binoculars, the exit pupil is 20 mm / 8X magnification is 2.5 mm

The exit pupil affects image brightness in that<ref>When looking at ordinary "extended objects," e.g. birds, targets, distant ships. The rules are different for astronomical point objects like stars</ref>

The exit pupil affects image brightness in that<ref>When looking at ordinary "extended objects," e.g. birds, targets, distant ships. The rules are different for astronomical point objects like stars</ref>

*The image is ''never brighter'' than the image that can be seen with the unaided eye.

*The image is ''never brighter'' than the image that can be seen with the unaided eye.

The pupil of the eye varies in size depending on how bright the light is. At night, the dark-adapted eye expands to about 7 mm. In room light or shady daylight, it is about 3-4 mm. In very bright daylight it may be less than 2 mm.  

The pupil of the eye varies in size depending on how bright the light is. At night, the dark-adapted eye expands to about 7&nbsp;mm. In room light or shady daylight, it is about 3–4&nbsp;mm. In very bright daylight it may be less than 2&nbsp;mm.  

What this means is that, in the examples above, night, the 7x50 "night glasses" will give a much brighter image than the other two. Under ordinary conditions, the 7x50 will not be any brighter than the 7x35s (and are considerably heavier and more expensive). And in bright daylight, there may not be any meaningful differences in brightness between the three (and if the 8x20s are slightly dimmer, that may be a good trade-off in return for their compactness and light weight).

What this means is that, in the examples above, night, the 7x50 "night glasses" will give a much brighter image than the other two. Under ordinary conditions, the 7x50 will not be any brighter than the 7x35s (and are considerably heavier and more expensive). And in bright daylight, there may not be any meaningful differences in brightness between the three (and if the 8x20s are slightly dimmer, that may be a good trade-off in return for their compactness and light weight).