Rifle Scopes

The Basics of Riflescopes And How They Work

Riflescopes, simply put, are devices that contain a series of lenses used for magnification and some type of reticle, an aiming instrument that indicates where your bullet should impact.

Scopes come in all sorts of configurations, styles, construction, sizes and price points. Sport shooters can (and do) argue endlessly about which scope maker or magnification or reticle is best for the job, but we’ll spare you the opinions, instead looking at the basics of how riflescopes actually work.

The most important components of a riflescope are its lenses -- the objective, erector, magnifying and ocular lenses. The objective lens is positioned forward in the tube away from the user. This lens transmits light to the erector lens, which then flips the image and sends it to the magnifying lens, which as the name implies, magnifies the image. From there, light travels to the ocular lens, which is nearest the user’s eye. The objective bell houses the objective lens, the erector and magnifying lenses live within the scope tube, and the ocular lens is contained within the eyepiece.

Evoking that old telescope you swore you saw UFOs with from your bedroom window, riflescopes work quite similarly. Light passes through the objective lens and focuses down into a point within the scope – the aptly named focal point. From here, the image is then inverted and captured by the erector lens or "picture reversal assembly." This system flips the image to the proper orientation, and sends light to the magnifying lens. The magnifying lens moves towards the objective lens while increasing the magnification, and rearward toward the ocular lens when lowering it. The ocular lens focuses the light gathered by the other end of the rifle scope, presenting the image you see when you look into the eyepiece.

A scope’s “eye relief” is the distance between the shooter’s eye and the eyepiece where the shooter can clearly see the entire field of view. In a shorter eye relief, the scope will be further toward the stock of the gun and closer to the user’s face. Conversely, long eye relief scopes, such as those employed on scout rifles, allow substantial room between the eyepiece and the eye, and can be mounted further forward on the firearm.

A majority of riflescope lenses are water and fog-proof, critical when considering the myriad conditions shooters will need to see clearly in. In order to waterproof an optical device, the manufacturer seals any gaps between the lenses and the housing.

Waterproofing a riflescope is accomplished by using O-rings, which seal and prevent moisture – or dust – inside the tube. While it’s great keeping water out of the optics chamber, waterproofing alone doesn’t prevent condensation – or fogging -- on the lenses, which can occur when moving from one temperature extreme to another (i.e. in a warm hunting lodge into a snowy, freezing field to hunt.)

Riflescope engineers overcome this by removing all of the air from the tube and replacing it with gas, such as nitrogen, argon or other combinations. These gases contain zero moisture, and therefore prevent condensation on the lenses.

The magnifying power of the scope indicates how many times the larger the field of view appears compared to the naked eye view. Some riflescopes are fixed power, meaning that you can’t adjust the magnification of the scope. An example of a fixed power scope is a 3X40, meaning the riflescope magnifies the field of view three times to that of the naked eye and uses a 40-millimeter objective lens.

The scope reticle can be placed either at the front or the rear of the magnifying lens. If the reticle is placed at the front of the magnification lens of a variable power scope, the crosshairs will change size when "zooming in." This will keep the target and the object in the scope the same relative size regardless of the magnification level. When the reticle is at the rear of the magnifying lens the crosshairs won't magnify with the object as the magnification is changed.

What are the Parts of a Rifle Scope?