How does a rifle scope work?

Optics are all about light. We all know that without it, we can’t see! However, if you think that light travels through a rifle scope like a tube with a magnifying glass in it… you probably need to read this. First of all, that would set your eyes on fire, and secondly, anyone could build one!

So, what do the guts of a rifle scope look like?

As the Canadians might say, let’s check ‘er oot.

A rifle scope is essentially a metal tube with glass lenses inside. Still with me? However, there are more than one or two, and they all have different purposes. In this article, we’ll look at the components of variable-power scopes, as those are the most common these days. The system within a fixed-power scope is essentially the same, save for a horizontally adjustable erector system.

So, how does light actually go from being reflected off of a target to being projected into your eye? Let’s follow it from one end to the other.

Light (most of it) comes from the Sun. That light reflects off of everything we can see. That’s the only way we can see it. The more light, the clearer we can see… to an extent. Too bright, and we become blinded as our eyes are overwhelmed and unable to adjust (pupil dilation) to the brightness (AKA amount) of that light.

Light was classically thought to take the form of a wave, and it technically does, but recent research has indicated that it is also a particle… sometimes. It can be both of those things at once via a theory called wave-particle dua…

Nevermind. We’ll save that for part two. Or never. Let the egg heads work that out. We can cover the basics of a variable telescopic optic system without an understanding of theoretical physics.

From One End to the Other

Light reflects off of a target in all directions, and you aim your scope at something to “capture” that light. Think of your eye as a camera sensor and your rifle scope as the lens in a cyborg-camera system. The scope performs a lot of invisible, highly technical functions that are necessary for a clear image and error-free shooting.

Objective Lens

The particles (or waves) of light enter the objective lens of the scope, which is as made as large as possible (read: practical) to capture more, and begin their journey to your eye. Due to the distance of the target from the objective lens, the image enters that lens upside-down. (To get a visual idea of this effect, look at your reflection in a spoon.)

Objective lenses are often coated with certain, mostly proprietary coatings made up of chemicals. These coatings filter certain types of light, reduce glare (reflection), and allow the maximum amount of light to be captured. Some coatings are selective for certain colors. 

This allows better performance in low light or grey, dusky conditions. Many Leupold rifle scopes are coated this way, and the color filters allow for greater resolution and differentiation of targets from environments in those lighting conditions.

The light passing through the objective lens creates a focal point. That focal point is where the reticle is located in a “first focal plane” or “front-focal” rifle scope. You will understand why this matters very soon. I promise.

Erector System

The next component is called the erector system. This consists of a series of two lenses whose purpose is to flip the image back to a right-side-up orientation. This is also where the elevation adjustment of the reticle takes place. When turned, the turret on the top of the scope activates a spring system that adjusts the image of the target, or in some rare cases, the physical position of the reticle itself.

Magnification

The magnification of the image is also performed by the erector system. The system’s two lenses (generally) move back and forth in synchronization with each other along a track that is machined into an inner, secondary tube. The lenses contract and expand, and the distance between them magnifies the image. 

These two lenses also create a second focal point (and plane) that may sound familiar. In a second focal plane or “rear-focal” scope, the reticle is situated on this point, between the erector system and the eyepiece.

Since it is located behind the erector system (responsible for magnification), the reticle is not magnified. In a first focal plane scope, the reticle is situated in front of the erector system, at the first focal point, which creates the first focal plane. Everything in front of the erector system is magnified by it. That is why FFP reticles “grow” with greater magnification. 

The longer the tube, the more space allowed for the two lenses of the erector system to move. The farther they are allowed to move away from each other, the more magnification the scope is capable of. 

That’s one reason that a scope like the Swarovski z8i 1-8x has a small form factor compared to something like the Leupold Mark 8 3.5-25X. If you are interested in the purpose of or differences between large and small form factor scopes, refer to our article on rifle scope weight for more information on the trade-off related to form factor and magnification.

Eyepiece/Ocular Lenses

After passing through the erector system and the reticle plane, light passes through two more lenses which allow the shooter to focus the image and the reticle to their eye. This is not the same as parallax correction. This final focal point is called the exit pupil and although it is usually referred to as “forgiving” or “unforgiving”, the distance of the exit pupil focal point from the ocular lens is based on the weapon it is mounted to.

On low-recoil rifles such as the AR-15 or other semi-auto carbines which often use lower magnification optics, the exit pupil can be closer to the lens and allow faster target acquisition. On high-caliber, bolt action rifles, the longer the exit pupil, the more “forgiving” it is, allowing the shooter some flexibility to find their ideal eye position and prevent the recoil of the rifle from having a surprising, violent introduction to their forehead.

There is a scar over my right eye from having that very encounter several years ago while hunting with a cheap rifle scope with a short exit pupil. Very important info to know when choosing the right rifle scope.

The most important and final part of an optical system is your optical sensors themselves. Take care of your eyes! They perform much more complex functions than any external optic and don’t come with an unconditional V.I.P warranty, although if they did, you could suspect that they were not made well. Wilderness Athlete sells a great product to help degenerating vision called Precision Vision, we advise checking it out.