Light Gathering

Light gathering capability is determined by the objective lens (the lens farthest from the eyes). The objective lens size is the second number used when describing a pair of binoculars. For example 8x35 refers to an objective lens size of 35 millimeters. A 50 mm objective lens will produce a brighter image than a 35 mm lens by letting in more light. This may be important to you if you do a lot of birding at dawn or dusk or in dense forests.

A pair of 8x35 binoculars will show a larger image than a pair of 7x50's, but the 7x50's will have a brighter image. Why is a bright image important to a birdwatcher? A bright image will show more detail, making it easier to identify the birds that you are looking at.

Hold your binoculars at arm's length and point them at a light source. The circle of light you see in the eyepiece is called the exit pupil. The size of the exit pupil indicates the binoculars’ brightness. You can calculate the size of the exit pupil by dividing the objective lens size by the magnification power. For the best results, only consider binoculars with an exit pupil of 5mm or larger.

Choosing the best birding binoculars will greatly enhance your enjoyment of watching wild birds, even if only at your backyard bird feeders.

Light gathering capability is important. Click here for more information on choosing binoculars for birding.