Optics 101

Every sportsman should have a good pair of binoculars and knowing what those crazy numbers mean is a must. Ask most sportsmen what the numbers 7x35 on their binoculars mean and I’ll bet they won’t know.

I’ll use a pair of 7x35 binoculars as an example. The first number given on a pair of binoculars tells you how much the object you are looking at will be magnified. In my example the object will appear 7 times closer than it actually is.

The second number tells you the diameter of the objective lens in millimeters. The larger the objective lens is the more light it allows in giving you a brighter sight picture. Of course a larger objective lens equals more weight.

You should also be aware of the eye relief that a pair of binoculars has, especially if you wear glasses. Eye relief is simply the distance from the lens, in millimeters, to a point where your eye is positioned to view the entire image. Most binoculars have an eye relief between 8-13 millimeters and a long eye relief pair, made for eye glass wearers, would be in the range of 14-20 millimeters.

Exit pupil is also an important number to know about. The amount of light that is usable by the binoculars is expressed as the “exit pupil”. A rule of thumb is the larger the exit pupil number the brighter the image will appear. Exit Pupil is arrived at by dividing the objective lens diameter, the second number, by the lens magnification, the first number. In our example the exit pupil would be 35 divided by 7 = 5.

Field of view, resolution and coated lenses could also be discussed but if you can master the above items you will be leaps and bounds above the average optics buyer.