The basic function of a scope is simple — it helps the shooter align the barrel of a firearm with the target. But how a scope manages to do that is slightly more complex. To better understand how high-tech optics work, we'll take a look at both the internal components and the external controls of a scope.
The inside of a rifle, shotgun or handgun scope works in much the same way as a telescope. There's an objective lens, which is located in the front of the scope and allows light to come into the scope body, and an ocular lens at the rear, which makes the sight picture visible to the shooter.
There are other internal lenses that play a role in how the scope functions, too. Behind the objective (front) lens, there's a focus lens, which helps focus objects in the scope.
Between the focus lens and the objective lens, there is an object known as an erector tube. The erector tube contains the magnification lenses and the reticle assembly (the reticle is what many shooters call the "crosshairs," which are used to aim the firearm).
The magnification lens moves toward the objective lens when increasing magnification. As the magnification is lowered, the lens moves closer to the ocular lens. So, if a shooter has a 3-9X scope, the magnification lens will be closest to the objective lens on 9X magnification and will be closest to the ocular lens on 3X.
The reticle can be mounted at the front or rear of the magnifying lenses, and this makes a difference in the image shooters see as they adjust magnification.
If the reticle is mounted at the front of the magnification lens (known as a "front (first) focal plane" scope), the reticle (crosshairs) will change size as the magnification is increased. This keeps the target and the object in the scope the same relative size, no matter what the magnification is set to. With a scope that has the reticle at the rear of the magnifying lens (known as a "rear (second) focal plane" scope), the reticle does not magnify with the object as the magnification is changed.
Most hunting scopes are rear focal plane scopes, while both first and second focal plane scopes can be found in the tactical optics market, the majority of the scopes in long-range applications being FFP. Front focal plane scopes make range estimation and long-range shooting simpler.
Lenses in a scope are oftentimes coated to reduce light reflection off the lenses. This increases the amount of light that is allowed to get through the optical system and into the eye, resulting in a brighter image. The internal tube is "purged" and sealed so that rain and moisture won't get in and fog the lenses.
External Parts of a Scope
The scope body is a metal tube that holds the lenses in place on the gun. The exterior of the scope also contains all of the necessary controls for making adjustments to the lenses to get a clear, crisp picture (like focusing a camera lens). Today, most scope tubes are made of aluminum, which is very light, durable and corrosion-resistant.
The front of the scope tube widens to accommodate the objective lens, and this flare is known as the objective bell. The rear of the scope widens to house the ocular lens, and this wider portion is known as the ocular bell.
The middle portion of the scope between the objective and ocular bells is called the main body tube. This is the portion of the scope where rings are attached to hold it to a firearm. Most main body tubes are 1 inch in diameter, though some are larger (30mm, 34mm or more). It's important to know the main body tube diameter when ordering rings to mount the scope, so be aware of these measurements.
On the main body tube of the scope, shooters will find two (or perhaps three) knobs for adjustments. These knobs are often capped on hunting scopes, and shooters may have to unscrew the cap to access the adjustment knobs. For most tactical and long-range hunting scopes, the adjustment knobs are easily exposed to the user. These are called "target turrets" and are advantageous for making many quick adjustments in the field or at the range.
When the scope is mounted on the firearm, the top knob is generally used to change elevation—how high or low the firearm shoots—and there is almost always a directional indicator to tell the shooter which direction increases elevation.
The knob on the right side of the scope is generally an adjustment for windage, which means how far right or left the projectile will strike. The windage and elevation knobs adjust the reticle and help shooters match the aiming point of the firearm with the crosshairs.
Most windage and elevation knobs have "click" adjustments you can feel when turning the dial. One click generally equates to one-quarter minute of angle adjustment and is usually written as 1/4 MOA.
Minute of angle, or MOA, is a ratio that remains constant at different distances. One minute of angle at 100 yards is almost exactly equal to one inch, while one minute of angle at 200 yards is about two inches.
This means that one click adjusts point of impact roughly 1/4 inch at 100 yards. If you rotate the windage knob three clicks to the left, the bullet will strike 3/4 inch to the left.
There are three other primary adjustments on your scope. The power ring is usually located just in front of the ocular bell and acts as the external control for the magnification lens inside the scope. Oftentimes there will be an ocular focus ring at the rear of the ocular bell, and this is rotated to help bring the reticle into focus for the shooter.
The last external adjustment — the parallax adjustment — allows the shooter to get the target and the reticle on the same focal plane.
To understand what it means to be on the same focal plane, imagine a needle speedometer in a car. If you are sitting in the passenger seat, the needle will appear to show a different speed than the car is actually traveling because of the angle of view. Adjusting parallax is like moving from the passenger seat into the driver's seat and giving you a clear view. This allows you to bring the reticle into focus with the target.
Many scopes have parallax adjustment, either an external knob on the left side of the body tube or a rotating ring on the objective bell.
Some scopes do not have an external adjustment and have a pre-set parallax. For most rimfire and shotgun scopes, that parallax is set at 50 yards. For centerfire rifle scopes, it is set at 100 yards.