Rifle scopes — What the numbers mean

by Tom Gaylord

On this scope the numbers are engraved on the objective ring at the large end of the optics.

You look at rifle scopes and they have lots of numbers in their titles and descriptions. Here is what they mean.

The primary job of a scope is to magnify the target, making it easier to see. So the magnification — how many times the image is magnified — is one of the numbers. If the scope is of fixed magnification or power, there is just one number. Four power means the image will appear four times larger than without the scope.

If the scope has variable power there will be two numbers, separated by a dash. The first is its lowest power and the second if its highest power. A 3-9 power scope has adjustable magnification between 3 and 9 times.

Objective lens size
After the power of the scope, there will be a capital X, like 3-9X50. The number after the X is the size of the objective lens in millimeters, so the example scope has a 50 mm objective lens. The objective lens is the one farthest from the shooter and closest to the target. The larger the objective lens, the more light can pass through and the brighter the image will be. As scopes magnify images more they need more light for the image to be clear. A 3-9X50 will be bright at all power settings, where a 3-9X32 will start getting dark as the magnification increases.

Tube size comes next
The next number you are interested in is the diameter of the scope tube. That’s the number you need to select the correct rings. This number isn’t written on the scope very often, but it’s always on the box and in the description.

Two sizes of scope tubes are common — one inch and 30mm. Thirty millimeters is larger than one inch and allows all the lenses inside the tube to be larger — meaning more light will pass through. So, 30mm scopes are usually brighter for a given magnification, and they are usually priced a little higher.

Scopes with 34mm tubes beginning to emerge in the market, though you won’t see many at the lower end of the price range for many years. But it’s a fact worth keeping in mind.

Illuminated scopes
Some scopes have illuminated reticles and their colors may be engraved on the scope. Red is the most common color, with green being a close second.

Adjustable objective
One last thing that isn’t a number. It’s the letters AO, which stands for adjustable objective. That means the scope can change the range at which its parallax is minimized. In variable power scopes that is a handy feature that wrings the last bit of accuracy potential from the scope.