Specifications for rifle scopes identify the rifle scope magnification and the diameter of the objective lens, measured in millimeters. There is a lot to unpack, so this article will focus on scope magnification as well as distance charts that help you visualize how the scope will work with your rifle.
Stated briefly, the first number in a rifle scope specification is the magnification of the scope. Thus, a 3×40 rifle scope has 3× magnification and a 40 mm objective lens. Objects viewed through the scope appear three times closer and three times larger than they appear to the naked eye. If a target is 300 yards away, the target will appear as if it were 100 feet away when viewed through the rifle scope.
In a variable magnification scope, the first number is a range. Thus, a 3-9×40 rifle scope can vary the magnification anywhere between 3× and 9×. This is analogous to a zoom lens. At its lowest magnification, the scope can magnify objects to three times their unmagnified distance. However, the magnification can be adjusted to zoom in and magnify the objects up to nine times their unmagnified distance.
The power of a scope may be represented graphically on a scope magnification distance chart. These charts illustrate the difference in the size of a target over various magnification powers at a fixed distance. Alternatively, a scope magnification distance chart may be expressed as a table that correlates three values – the apparent size of the target, the actual size of the target, and the distance to the target. If you know two of these values, this table will tell you the third value.
You might believe that greater magnification is always better. However, you cannot select scope magnification simply based on the size of the numerical value. Rather, several factors play into choosing the right magnification for your rifle and your intended use.
Select a Rifle Scope Magnification Based on the Distance to Your Target
The first factor is choosing a scope magnification is the anticipated distance to your target. If you need a scope strictly for the shooting range, this distance will be known. If you hunt game, you need to extrapolate a distance based on the type of game. For example, the distance of most shots taken while hunting deer will fall between 100 and 200 yards, with the occasional shot up to 400 yards.
While there is no hard and fast rule, shooters usually find that they need no more than 10× magnification up to 500 yards, with many settling in at 4× or 6× magnification. The reason many stop at that number is because choosing a higher magnification than necessary produces diminishing returns.
High magnification produces a larger apparent image. But this magnification comes at the cost of resolution because high magnification scopes are more prone to distortion and loss of detail. The result is a choice between a highly magnified, but distorted, image of a deer’s nose or a less magnified, but sharp image of the deer’s body.
Moreover, high magnification amplifies any motion of your rifle. At a high enough magnification, you may even see the image in your scope move with your breathing and heartbeat.
Finally, high magnification scopes are susceptible to mirage effects. That is, temperature gradients in the air between you and the target can distort the image. Viewing these distortions through a high-powered rifle scope will magnify these mirage effects and make the target appear wavy or even create a double image of the target.
Moving Targets Require Lower Magnification Than Stationary Targets
Another factor in choosing the right magnification is the anticipated field of view that you will require from your scope. Field of view is simply an acknowledgement that you can see a smaller area through a high magnification scope than you can through a low magnification scope.
This is logical. If your scope is so powerful that you can zoom in on your target’s nose, it only needs to move its head a few inches before it disappears from your scope’s field of view. A lower magnification scope, on the other hand, might include within its field of view the target’s entire body as well as its surroundings. You may even spot a better target nearby.
Field of view is particularly important when tracking moving targets. Once sighted, stationary targets, by definition, will not move outside your field of view. Moving targets, on the other hand, can be difficult to pick up and track with a narrow field of view. When you are searching for a flash of motion, you are much more likely to spot it in a wide field of view with less magnification than a narrow field of view with greater magnification.
Images also appear dimmer in scopes with high magnification than scopes with low magnification. If you compare the field of view to a tube, a narrow tube collects less light than a wide tube. It naturally follows that the image through a narrow field of view will be dimmer than the image through a wide field of view. The flash of motion associated with a moving target can be lost in a dim image.
Match the Scope’s Magnification to Your Rifle’s Range
Yet another factor in selecting a rifle scope is the rifle. As noted above, many hunters find that shots up to 500 yards require no more than 10× magnification. It makes sense, then, that you would not mount a high-magnification scope on a rifle that lacks the range or accuracy for shots longer than 500 yards. For example, a scope with more than 10× magnification would be wasted on a .22 caliber rifle or .30-30 caliber rifle with their effective ranges of about 200 yards.
This can be an important consideration for many shooters because scopes with higher magnification are heavier than scopes with lower magnification. This added weight can unnecessarily throw off your balance while shooting and lead to fatigue as you carry the extra weight.
Prioritize What You Want When Selecting a Rifle Scope Magnification
Choosing the right rifle scope magnification is not a matter of picking the biggest number. Rather, it is about selecting a magnification that matches your target range, target type, and rifle.