Choosing Binoculars

The power of bird watching binoculars is not a measure of quality. Rather, it is only one of several factors to consider. Following is a brief overview of some of those factors.

What do the numbers mean?

Bird watching binoculars are labeled 7x35, 8x40, 10x50, etc. The first number gives magnification. For example, an object 80 yards away will appear 1/8th of that distance, or only 10 yards away with 8x40 glasses. The second number 40, is the diameter of the objective lenses (the large lenses on the front of the bird watching binoculars) in millimeters. These numbers combined give the light generating power of the bird watching binoculars - how bright an image appears though the glasses.

Exit Pupil (EP)

This is the most common measure of the binoculars Light generating power mentioned above. By dividing the objective size (second number) by the magnification (first number), you determine exit pupil. The larger the exit pupil, the more light is getting through to your eyes.

Field of View (FV)

This measures the diameter (measured in feet at a distance of 1,000 feet away) of the area seen through the glasses. For example, 375 means that you can see an area of 375 feet through the glasses when standing 1,000 feet away.

Eye Relief (ER)

If you wear eyeglasses, using bird watching binoculars should not seem like you're looking through a tunnel which restricts the field of view. Since your eyeglasses cause your eyes to be further from the ocular lenses (lenses closest to your eyes), this "tunnel vision" effect will occur with improperly fitted binoculars. Eye Relief is the distance (in millimeters) that your eyes can be from the ocular lenses and still see the full field of view.