Any gun enthusiast will tell you that the scope is one of the game-changing additions to modern rifles. There’s no denying that adding high-end optics to a hunting gun, a designated paper puncher, or a military rifle will tremendously improve your precision.
But despite the wide range of rifle scopes available in the market, many gun enthusiasts still struggle when choosing the right piece for their weapons. Maybe it’s the jargon that comes with gun optics or maybe it’s the complexity of understanding the various features of rifle scopes and how they affect your shots.
But one thing is for sure: many people don’t understand how rifle scopes are measured. But worry not, you are about to learn what the numbers on your riflescope mean, and how they affect your shooting accuracy.
How Rifle Scopes are Measured: Key Measurements
1. Magnification and Objective Lens
This is the most important numerology on a rifle scope. It describes the magnification and the size of the scope’s objective lens.
If you take a closer look at your scope, you will notice some writing on the side of the scope – for example, 3-9x40. This means that the scope’s magnification is somewhere between 3-power and 9-power. The number 40 represents the diameter of the objective lens, usually in millimeters. Such a riflescope would be described as a “three to nine by forty”.
The larger the diameter of the objective lens, the clearer and brighter your target will appear. Also, the larger the objective lens, the more expensive the optics.
Sometimes, the objective lens of your riflescope will not perform as advertised. Fortunately, there’s a four-step test you can use to find out if your optic’s lens is as good as what’s written on it.
For this test, you will need an extra pair of hands, a white paper sheet, a pencil, an incandescent lamp, and a table. Once you have everything in place, follow the following steps:
- Remove the shade of an incandescent lamp and place the lamp on a table.
- Grab your optic and hold the eyepiece against the lamp such that light streams through the lens.
- Hold the white piece of paper on the other end the objective lens. You will notice a bright light disk surrounded by a dark circle. Have your partner precisely mark the edges of the light disk.
- Measure the marks made and compare them to the dimensions of your rifle scope.
The measurements should either match or differ by not more than two millimeters. For example, if you were testing a 40mm scope, the paper marks should measure anywhere between 38mm and 40mm.
If the disc of light reflected by your optics is way smaller than the dimensions of your objective lens, it means that the manufacturer has dialed down on the objective lens. Sometimes, the objective lens may truly be 40mm in size, but the metal discs that connect to the scope may block the edges of the lenses and result to a smaller dimension.
2. Field of View
The field of view in a rifle scope is simply the size of the field vision a shooter can see at a certain distance. It’s measured using feet or degrees per yardage.
For instance, a scope set at 100-142 ft @ 1000 yards means that the shooter will see 100-142 feet wider than they would see with their naked eye.
Generally, moving targets require scopes with larger fields of view than still targets.
3. Tube Diameter
The tube diameter is the diameter at the middle of your scope’s tube. While a bigger tube diameter does not change the quality of optics, it can affect the height at which you can adjust your scope from your rifle.
4. Eye Relief
This is the distance between your eye and the scope’s eyepiece that will allow you to see the whole field of view. It is usually measured in millimeters.
Generally, low power riflescopes have a longer eye relief compared to high power optics.
5. Exit Pupil
Usually in millimeters, this specification is used to measure the diameter of the light circle that meets the eye of the shooter. A small exit pupil gives the shooter a dim image, while a very large exit pupil wastes a lot of light.
A 4-5mm exit pupil is optimal for low light conditions, while a 6mm exit pupil is suitable for dark environments. Not all riflescopes bear these specifications, so check carefully before you buy.
The total weight of your rifle scope is usually indicated in ounces or kilograms. It is an important factor to consider as it affects the balance of your rifle and the ease of wielding it.
Now You Know!
Now that you know how rifle scopes are measured, you can make informed decisions when buying optics for your rifle. Let these six measurements guide your choices, budget, and type of rifle you want.