Light-gathering Capacity
To a birder, binoculars' light-gathering capacity is nearly as
important as image sharpness. Only a bright image reveals subtle field
marks and the full beauty of bird colors.
Light enters binoculars through the objective lenses, those farthest from the eye. The diameter of these lenses in millimeters is the second number in the binoculars' designation, so 7X35 binoculars, for instance, have 35mm objective lenses. The bigger the objective lenses, the more light gathered and the brighter the image. 7X50 binoculars have the same magnification power as a pair of 7X35s, but the 7X50s, with their 50mm objective lenses, have greater light-gathering ability. Just as an owl's large eyes gather sufficient light to allow nocturnal vision, binoculars with large objective lenses permit birding in low light, such as dawn or dusk, or in dark forests.
The best measure of a binocular's brightness is the size of the exit
pupil, which you can see by holding your binoculars at arm's length and
looking into the eyepieces. A small exit pupil appears as a dark hole,
whereas a large one is a brilliant, clear circle.
To calculate the exit pupil size, divide the objective lens diameter by
the magnification power. For example, 7X35 binoculars have an exit
pupil of 5mm, whereas 7x50s have an exit pupil of 7.1mm, which provides
a much brighter image. Avoid binoculars with an exit pupil of less than
5mm, which give a dim image, useful only under bright lighting
conditions.
