An excellent scope is a vital part of anyone's rifle.
Whether for target plinking or competition long range shooting, knowing your rifle scope is extremely important to your performance. Because there are so many different options for scopes, here’s how you can identify specific features of a scope, and ultimately decide on which optic will suit your needs. In this article, we’ll be going over 3 main features:
- Units of adjustment
- Parallax
- Focal planes
- And a brief discussion of eye relief.
First of all, it’s important to understand the units of measurement for scope adjustment.
There are two unique methods of measurement: MRAD (or MIL), and MOA. Both measurements both relate heavily to trigonometry, so if you’re familiar with trigonometry and basic algebra, you’re in for a treat.
All scopes are designed to have their click adjustments measured in either MRAD or MOA. Knowing how to use each will greatly assist you in your shooting experience.
MRAD or MIL: Both MRAD and MIL are short for milliradians (1 radian = 1000 milliradians). This means that one click on your scope will span an angle in milliradians (see illustration below).
Simply put, one click on a 1 MRAD scope will move the point of aim 3.6* inches at 100 yards.
*diagram says 3.7 inches; 3.6/3.7 inches are interchangeable
MOA: MOA is short for minutes of angle. Angles in imperial measurement are typically measured in degrees. Mathematically, angles are split into degrees, degree minutes, and degree seconds (DMS). MOA uses degree minutes; hence, “minutes of angle”.
As with time, 60 degree seconds is equal to 1 degree minute, and 60 degree minutes is equal to 1 degree. If it helps to think of these angles in terms of degrees, 1 MOA is equal to 1/60th of a degree, or ~0.06667 degrees.
This is how MOA adjustments look at 100 yards:
Simply put, one click on a 1 MOA scope will move the point of aim 1 inch at 100 yards.
Why is this important?
Knowing the unit of measurement on your scope will help you with zeroing and adjusting between shots. As long as you know what adjustment value your scope is (more on that later) and the distance to your target, you have a basic idea of how to adjust.
To put this in perspective, let’s set a scenario up with MOA, the arguably more common of the two units of measurement. If you’re 100 yards away from your target, and you have a 1 MOA scope, each click on your scope will move your reticle* about 1 inch in whatever direction you choose, as seen in the MOA diagram above. If your bullet hits about an inch above your reticle, you can move the reticle up by one click on your scope and your point of impact will now be dead on your reticle. Similarly, if your bullet hits an inch to the right of your reticle, you can move the reticle to the right by one click and your point of impact will now be dead on your reticle.
*A small note on zeroing your optics – scopes use one of two adjustment concepts; adjusting the reticle, and adjusting the bullet. For the “adjusting the reticle” concept, you adjust the scope as if you’re moving your reticle to the bullet’s point of impact. For the “adjusting the bullet” concept, you adjust the scope as if you’re moving the bullet’s point of impact to your reticle. Yes, you’re really just adjusting the reticle in both cases, but these concepts are just an illustration to help the user understand how their scope works. The “adjusting the bullet” concept tends to be more common.
When you first get a scope, fiddle around with it and see if the adjustments “adjust the reticle” or “adjust the bullet”. Keep this in mind when zeroing any scope, as each scope is different.
The second feature we will be examining is parallax.
Parallax is an effect that affects perspective when looking at objects from different distances. Parallax is what creates depth of field; that is, when you focus on one object with your eyes, the surroundings are blurry. This is utilized to great artistic effect in photography and film.
Here are some examples.
The photo at right demonstrates short depth of field.
Notice how only the ladybug is in focus, while its surroundings are blurred.
The photo at right demonstrates long depth of field.
Notice how the entire picture is in focus with no background blur.
What does this mean for a rifle scope?
All instruments with lenses are subject to parallax, including the human eye.
A good parallax adjustment on a scope is similar to a long depth of field; that is, all objects in the scope, whether far or near, are in focus. Think of far objects as your target, and close objects as the reticle itself. In this way, a good parallax means both your target and reticle are crystal clear at the same time.
Although not entirely accurate, you can think of parallax adjustment as adjusting focus, since you are trying to find a balance in clarity between far objects and near objects.
Another way of adjusting parallax is with adjustable objective, or AO, scopes. Adjustable Objective simply means you adjust the focus based on your range. If your target is at 100 yards, you would adjust the objective to 100 yards, and your scope should have perfectly focused parallax for 100 yards.
To put it simply, a good parallax makes sure you can actually see your target clearly.
The third feature we will be examining is the focal plane.
Remember how we talked about how you could think of the target as a far object, and the reticle itself as a close object? Well, it turns out that there is a system loosely based on this example that describes what a scope does when you zoom in. This system is called focal planes.
The concept of a focal plane is demonstrated below:
The idea of a focal plane treats foreground and background as if they are two layers sitting on top of each other. Running with our previous analogy, whatever sits on the blue (first/front) plane is a “close” object, and whatever sits on the red (second/rear) plane is a “far object”.
What does this have to do with scopes?
There are two types of scope focal planes: FFP and SFP.
FFP (First Focal Plane): Also called Front Focal Plane (FFP), FFP refers to the blue plane in the diagram above. FFP simply means “When you zoom in, the reticle zooms in as well.” See below for a visual explanation.
SFP (Second Focal Plane): Also called Rear Focal Plane (RFP), SFP refers to the red plane in the diagram above. SFP simply means “When you zoom in, the reticle stays the same.” See below for a visual explanation.
Why is this helpful?
Believe it or not, zooming in on your scope can actually change where your rounds hit, potentially costing you a hit on your target:
With the example above, even though your scope may be zeroed when fully zoomed out, your point of impact may immediately change when you zoom in.
Let’s say you wanted to aim based on the dot markings on the reticle.
Imagine you’re zooomed all the way out. If you want to aim with the dot directly to the right of the crosshairs, you would hit pretty far from the “bullseye”. But if you were to use that same dot when zoomed all the way in, you would hit a near-bullseye.
Going back to FFP and SFP:
An FFP scope enlarges the reticle as you zoom in. This means that the distance in between dots is always the same; you won’t have to worry about it changing from zoom.
Notice how the reticle enlarges as you zoom in. When zoomed out, the whole target is only around 4 dots wide. When zoomed in, the target is around 8 dots wide*.
*Note: The illustration below is merely conceptual and does not accurately depict real-life proportions. If this was truly accurate, the target would still be 4 dots wide, even after zooming in.
On the other hand, an SFP scope keeps the reticle the same as you zoom in. Although this is simpler, it does mean you have to worry about whether or not your scope is still accurate when you change zoom.
Notice how the reticle stays the same as you zoom in. When zoomed out, the target is only around 4 dots wide. When zoomed in, the target jumps to 10+ dots wide.
The fourth and final feature we will be examining is eye relief.
Eye relief is the distance between your eye and the scope eyepiece. This distance is usually designed by manufacturers to be 3-4 inches.
The point of eye relief is to maximize the picture inside your scope, while still giving you good peripheral vision to stay situationally aware.
Having your scope too close or too far away from your eye fails to maximize the picture you see in your scope, which looks like the image at right.
A good eye relief maximizes the picture you see in your scope, as seen at right.
Eye relief is usually 3-4 inches to give you good peripheral vision while also preventing the scope from hitting your eye when you fire.
However, each scope is made differently; make sure to find a scope that has an eye relief that suits your needs, based on your mission.
In this article, we covered units of adjustment, parallax, focal planes, and eye relief.
- Scope adjustments are measured in terms of MOA and MRAD. Just remember that 1 MOA means “One click will move 1 inch at 100 yards” and 1 MRAD means “One click will move 3.6 inches at 100 yards”.
- Parallax is typically adjusted based on your range. Know what the minimum parallax of your scope is (meaning, “This is the minimum distance that both your target and reticle are clear at the same time”), and know what your range is to better adjust your scope.
- FFP means “When you zoom in, the reticle zooms in as well”, and SFP means “When you zoom in, the reticle stays the same”. When picking out a scope, decide whether you want FFP or SFP, as each carries unique advantages and disadvantages.
- Eye relief is the distance between your eye and your scope. You want this distance to be on par with the factory determined distance to ensure that you get maximum picture in your scope, while at the same time maintaining good situational awareness, and of course, not getting hit in the eye with your scope when you fire.
Each of these features may greatly assist your shooting experience if tuned correctly. Find what you like, work with it, and enjoy the process. With practice, you’ll be driving tacks in no time!
