The most important parts of the rifle scope are the lenses. The larger lens is the objective lens. The objective lens is on the end of the scope farthest from the rifle's stock. Its purpose is to transmit light back to the ocular lens, which is the lens closest to your eye. The part of the scope that houses the objective lens is the objective bell, while the section containing the ocular lens is the eyepiece. Most rifle scope lenses are waterproof and fog-proof.
Rifle scopes work like telescopes. Light passing through the objective lens focuses on a point inside the scope. The ocular lens magnifies the light from the focal point. When you look through a scope, the image you see is that light. Rifle scopes also have a reticle, also known as a crosshair. The purpose for these markers is to show the shooter exactly where the shot will go once he or she pulls the trigger.
Some scopes have multiple settings that allow you to view targets at different magnifications. For example, a scope might allow you to view targets from 3x to 9x your normal vision. That means if you set your scope to 3x magnification, any object you view through the scope will appear three times larger than if you looked at it without the use of the scope. These scopes have a feature called a power ring. Turning the power ring changes the magnification setting on the scope.
Most manufacturers set their rifle scopes so that they're focused at 100 yards (91.4 meters). That means when aiming at an object 100 yards away, the target should be clear. But switching magnification settings can introduce parallax error. With rifle scopes, parallax error is when the aim on a scope changes if the position of the shooter's eye changes. The rifle can remain perfectly still, but by shifting your position it will look like your aim is off target. Parallax error becomes a problem at high magnifications -- most hunters won't ever have to worry about it. Some manufacturers build rifle scopes with adjustable objective lenses that can correct for parallax error.
Rifle scopes also have a few controls that allow you to adjust the scope so that it's in alignment with your rifle. The two controls that affect a scope's sight are the windage adjustment and the elevation adjustment. The windage adjustment tweaks the horizontal settings on a scope, while the elevation adjustment changes the vertical settings.
The main body of the rifle scope is the tube. There are two main diameter sizes of tubes for rifle scopes: 1-inch tubes and 30-millimeter tubes. It's important to know the diameter of your scope's tube so that you use the correct mounting rings when you attach the scope to your rifle.
Now that we know the basic anatomy of a scope, let's look at the different kinds you can buy for your rifle.