Choosing a scope for your rifle can be a big decision. A scope is no small investment, whether you opt for a top-of-the-line model or an affordable option for a couple hundred bucks.
Luckily, most optics manufacturers give you all the information you need to be able to compare models and find the one that’s best for you. The catch is, you have to be able to interpret it.
Most rifle scopes, after listing the make and model, will have a line of text that looks something like this: 3-9x50mm.
Those three numbers represent the minimum and maximum magnification levels and the objective lens diameter. So in the above example, the scope would have a minimum power of 3x magnification and a max of 9x, and a large 50mm objective lens.
Each of these numbers offers critical information you can use to compare one optic to the next. You’ll find even more numbers available in the details of the product.
To help you decipher what all these numbers mean and which ones are important for you, we’ve put together this handy guide. Below, we’ll tackle each metric one by one.
Magnification
The first number in the above 3-number string is always the magnification power. This is a measure of how many times the optic amplifies what you’re looking at. For a 3x optic, for example, anything viewed through the optic would appear 3 times as large as it would with the naked eye.
In the case of fixed power optics, there will just be one number given, so instead of a 3-number string, you’ll see something like 3×24 or 4×32. For variable zoom optics, the power range is written as two numbers separated by a dash. If we refer again to the above example, a scope that is listed as 3-9x50mm would have a magnification range that is adjustable from 3x to 9x power allowing you to set the effective magnification level anywhere in between those two numbers.
Magnification is one of the first things you’ll want to consider when selecting a scope. Too much magnification will make it difficult to engage targets quickly at close range. Too little will make it hard to see targets at longer distances.
Objective Lens Diameter
The last number in the series is the objective lens diameter. The objective lens is the one at the front of the scope, towards the muzzle, not the lens you peer through.
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All else being equal, objective lens size correlates directly with image brightness. The larger the lens, the more light is gathered by the scope, rendering a brighter image. This is especially useful in conditions where ambient light is low, such as around dawn or dusk, or when shooting into a shaded area.
Objective lens size will also determine the minimum height at which your scope much sit on your rifle. Large 50mm or 56mm objective lenses will require taller rings.
Maintube Size
Also known as “tube size” or “body diameter,” this specification is usually found in the description or details of the scope, but you may sometimes see it listed after the magnification or objective lens diameter.
Maintube size refers to the diameter of the main housing of the scope. Common sizes include 1-inch, 30mm, and 34mm, with the first two being more common than the last.
The primary effect of the maintube size is on the internal adjustment range of the scope. A larger maintube has more room inside, allowing the turrets to be adjusted farther before maxing out. As such, larger tubes are particularly important for long-range rifles in calibers with steep ballistic drop profiles.
For most users, internal elevation adjustment is not typically a concern, as even the average 1-inch or 30mm scope usually has enough adjustment to take a common cartridge like .223 Remington or .308 Winchester out to its maximum effective range. However, if you’re scope shopping for a rifle chambered in .50 BMG, it’s much more relevant.
Another effect is on mount availability. Scope mounts and rings are commonly available for the aforementioned sizes, but for less common tube sizes like 3/4″ or 35mm, your options will be considerably more limited.
Exit Pupil/Eye Relief
While these are technically two different measurements, their practical application is very closely related.
Eye relief is the distance from the ocular lens of the scope (the lens you look through) to your eyeball at which the scope can be properly used. For most scopes, this will be listed as a range, usually somewhere from 2″-5″.
So long as your head is positioned so that your eye is within that range, you’ll be able to see through your scope properly. Too close or too far, and you’ll see a blurry black ring around the edge of your optic and you’ll have a hard time picking up your reticle. Obviously, a larger range is better, as it will allow for a greater margin of error when using your rifle.
Exit pupil is a more complicated concept, but you can think of it as a small circle behind the scope, through which all of the light leaving the scope passes. As long as your eye is inside that circle, you can see the image through the scope. If you’re out of alignment with the scope, you’ll see a partial black ring on one edge.
As with eye relief, a larger exit pupil is generally better, as it will be more forgiving.
Combined, these two measurements form what we call the eye box, which can be thought of as a cylindrical space behind the scope. So long as your eye is in the box, you’ll have a good sight picture.
While a larger eye box is preferred, there are always trade-offs. Often high-powered scopes will have a smaller eye box than low-powered variable optics.
This is technically to their deficit, but because high-powered scopes are generally used on long-range rifles fired from stable positions, the eye box is less crucial. An LPVO, on the other hand, which is frequently used to engage targets rapidly or while moving, benefits much more from a forgiving eye box.
Parallax Adjustment
While you won’t usually see parallax adjustment listed as a number in the scope’s specifications, you will see a range of numbers on the adjustment knob, if your scope has one.
Parallax refers to an optical phenomenon that can cause your reticle to become misaligned with your point of aim. We won’t get into the science of it all, but the gist of it is, if your target is located substantially beyond the parallax setting of your scope, then any misalignment between your eye and your scope can cause your crosshairs to appear misaligned.
Luckily, many scopes come with adjustable parallax, allowing you to recalibrate your parallax setting on the fly to make it match your distance to the target.
Often, you’ll find the parallax adjustment knob located on the left side of the scope housing, opposite the windage turret. However, just because your scope has a knob there doesn’t mean it has adjustable parallax. That’s a popular location for an illumination adjustment knob as well.
When shopping for scopes, check the specifications to see if the parallax is adjustable, and the range of adjustment offered. Typically, scopes designed for long-range precision rifles will have adjustable parallax, while LPVOs and prism scopes will not.
Wrap Up
Those key numbers will tell you most of what you need to know about a rifle scope. There are always going to be certain hard-to-quantify variables, such as glass clarity and color fidelity, as well as other factors like reticle design, but those numbers will allow you to compare the raw specifications between models.
Of course, if you’re shopping for something more sophisticated like a night vision or thermal scope, you’ll have to learn to decipher even more numbers and metrics.
But we have guides for those as well. Head over to our Ultimate Night Vision Guide and Guide to Thermal Optics if you’re interested in finding out more.
Whatever scope you end up shopping for, start by determining your preferred magnification and objective size for your application, then move on to finer details like maintube size and eye relief.
