Rifle Scope Anatomy and Terminology

Rifle Scope Anatomy

When asking the question “How to choose a rifle scope”, it is important to understand the basic parts of a rifle scope and their function.  To help with this understanding, the following diagram illustrates rifle scope anatomy and contains links to descriptions of the different parts of a scope.

Eye Bell

The major section on the end of the scope that you look into.  It is the housing that the eyepiece and the tube connect to.

Eye Relief

The distance at which your eye can be from the eye piece to get a full field of view.  Typically the larger the better, especially with higher caliber rifles as you need the distance to compensate for the amount of kick in the gun while keeping the scope from smacking back and hitting you in the face.

Ocular Lens

The ocular lens is the lens closest to your eye where you actually look into to see the images that are magnified by the scope.  The exit pupil is emitted from this lens.

Eyepiece

The piece on the scope that surrounds the ocular lens and attaches it to the eye bell.

Power Ring

An adjustable piece on the eye bell of variable power scopes that allows the user to adjust the magnification.

Tube

The tube of a rifle scope is the portion of the scope where a series of lens are setup and adjusted to bend the light through the scope to magnify the images for the user.  There is a tube within the tube when it comes to a rifle scope.  The tube you see from the outside is really the outer tube, serving to protect the internal parts.  The inner tube is where the lenses are housed that are able to move freely to adjust magnification of the rifle scope.

When shopping for a rifle scope, you’ll want to make note of the tube diameter as it has a direct impact on the size of rings you purchase for your scope.  Most tubes come in 1″ diameter, however, some higher end and higher magnification scopes come in 30mm tube diameters.  With a higher magnification rifle scope, the 30mm diameter tube is often preferred as it allows the scope to have a wider range of windage and elevation adjustments due to the extra space the lenses can move.

Windage Adjustment

The windage adjustment allows the user to adjust the aiming point of the rifle on the horizontal plane(side to side). This adjustment is used in combination with the elevation adjustment to allow the shooter to zero in their rifle.

Adjustments are typically made in increments of 1/4 or 1/8 MOA(Minute of Angle), determined by the specific characteristics of the rifle scope.

Elevation Adjustment

The elevation adjustment allows the user to adjust the aiming point of the rifle on the vertical plane(up/down).  This adjustment is used in combination with the windage adjustment to allow the shooter to zero in their rifle.

Adjustments are typically made in increments of 1/4 or 1/8 MOA(Minute of Angle), determined by the specific characteristics of the rifle scope.

In addition to being used to zero in the rifle, elevation adjustments are often used to compensate for bullet drop at yardages well beyond the distance the scope is sighted in at(typically 100 yards).  The shooter can use the elevation adjustment to move the aiming point up or down on a target at known distances to allow them to aim by putting the cross-hairs directly on the target.

Some manufacturers, such as Leupold, will custom make special elevation adjustment dials that are specific to your ammunition as well as the specific ballistics of your rifle.  This allows these shooters to simply turn the elevation dial to “300 yards” when shooting at a target 300 yards away, and then put the cross-hairs directly on the target and have an accurate shooting solution.

Objective Bell

The piece at the end of the scope from your eye that houses the objective lens.

Adjustable Objective

An adjustable objective is a feature that is found on many higher powered rifle scopes.  In most cases it is a dial around the objective bell or end of the scope, or less frequent, but growing in popularity, a knob on the left side of the turret housing.  The function of the AO is to allow you to adjust your scope’s parallax to a certain distance by moving these adjustments until a clear picture is perceived, and/or the marked corresponding incremental yardages are approximated to your target distance. The correct setting of an adjustable objective prevents the apparent movement between the reticle and the target when the shooter moves his head slightly off center of the rifle scope, other wise known as parallax error.

Objective Lens

The objective lens is the lens that is situated towards the barrel end of the the rifle.  It is the lens from which the light enters the scope.  It is measured in millimeters(mm) in diameter.  The size of the objective lens determines the amount of light that is able to enter the scope.  The larger the objective lens, the more light that is able to enter the scope.  Longer range scopes generally have larger objective lenses for this reason as they need more light to enter the scope to allow for bright images at higher magnification settings.  Objective lenses are also the major determinant of the height of the rings needed to mount your scope.  The larger the objective lens, the higher rings you need to allow your lens to clear the barrel of the scope.  With a typical scope description of 3-9X40, the objective lens is 40mm.

Adjustable Objective

An adjustable objective is a feature that is found on many higher powered rifle scopes.  In most cases it is a dial around the objective bell or end of the scope, or less frequent, but growing in popularity, a knob on the left side of the turret housing.  The function of the AO is to allow you to adjust your scope’s parallax to a certain distance by moving these adjustments until a clear picture is perceived, and/or the marked corresponding incremental yardages are approximated to your target distance. The correct setting of an adjustable objective prevents the apparent movement between the reticle and the target when the shooter moves his head slightly off center of the rifle scope, orther wise known as parallax error.

Rifle Scope Terminology

Coatings

Microscopic coatings on the lens surfaces reduce light loss and glare due to reflection.  Necessary due to the fact that a portion of light is reflected or not transmitted by each air to lens surface barrier that is crossed.  Lenses with coatings minimize the amount of light that is reflected or otherwise not transmitted through the lens, resulting in an image that is brighter, clearer, with higher contrast.  The more coatings on a particular lens(multicoating), the wider array of different wavelength light rays that can be transmitted, resulting in an even better picture and higher transmission.  Not all coatings are made equal, so having a “multicoated” lens does not always mean that it will outperform a single coated lens.  There are factors to consider such as the coating material as well as the quality of the glass.

Coating Designations

Click

A click is an adjustment, usually measured in MOA, on the windage or elevation turret of a scope.  A single click often adjusts scope’s point of impact by ¼ MOA(or ¼ inch at 100 yards).  However, some other scopes have adjustments that are set at ⅛ MOA or 1/2MOA.

Exit Pupil

An exit pupil is the small circle of light that emits from the ocular lens when you hold your scope at arm’s length. The larger the exit pupil is, the brighter the image you will see. To determine the size the exit pupil for your scope, divide the objective lens diameter(in mm)by the magnification. For example, if your scope is four power (9X), and your objective lens is thirty-six millimeters in diameter (44mm), divide 9 into 44 and it equals 5(roughly). Five would be the exit pupil size in diameter in millimeters.

Eye Relief

Eye relief is the distance your eye must be from the ocular lens and still get a full field of view. Typically 3.5” or above is desired.

Field of View

Field of view (FOV) is the amount of area seen through your scope from right to left at 100 yards. As magnification is increased, FOV is lessened. As magnification is decreased, FOV is increased. For example, a typical 3X variable scope might have a FOV at 100 yards of a bit over 30 feet, and at 9X, the FOV would be around 14 feet. A larger objective lens will not change these figures.

Hold Over/Under

Hold over/under is the amount of point of aim change either above or below your target, without adjusting your scope, to adjust for the trajectory of your projectile.  This is typically done by re-adjusting your aim such that the crosshairs are actually above the target when accounting for bullet elevation or wind adjustments at longer ranges without the use of MOA clicks on your scope.

Kentucky Windage

Similar to Hold Over, this is simply the amount of point of aim change either left or right of your target, without adjusting your scope, to adjust for windage.

Light Transmission

Light transmission is a number, represented as a percentage, of the amount of usable light that passes through a scope, binocular, etc, based on the amount of original light that entered the instrument.  Light Transmission for a high quality rifle scope is 94% or better, meaning that of the original light that entered from the objective lens, 94% of that light is preserved and transmitted from the exit pupil.

Magnification

Magnification is the number that denotes how powerful your scope is.  The number itself represents how much larger the scope will make the target than the naked eye.  It is indicated by the number to the left of “X” on your scope description.  A “9X” scope will maginfy the image in the scope 9 times larger than capable of being seen by the naked eye.  There are variable power scopes, denoted by descriptions such as “3-9X” that are capable of magnifying the images in the scope as low as 3 times as large as with the naked eye or up to 9 times larger.

Minute of Angle

Minute of Angle (MOA) is a unit of measurement of a circle, and is 1.0472 inches at 100 yards. For all practical purposes it is called 1 inch at 100 yards. It is 2 inches at 200 yards, 5 inches at 500 yards, one half inch at 50 yards, etc.  Windage and Elevation turrets on scopes usually correlate with MOA, and are most most often set to adjust MOA at the rate of ¼ MOA per click, or adjustment.

Objective Lens

The objective lens is the lens that is situated towards the barrel end of the the rifle.  It is the lens from which the light enters the scope.  It is measured in millimeters(mm) in diameter.  The size of the objective lens determines the amount of light that is able to enter the scope.  The larger the objective lens, the more light that is able to enter the scope.  Longer range scopes generally have larger objective lenses for this reason as they need more light to enter the scope to allow for bright images at higher magnification settings.  Objective lenses are also the major determinant of the height of the rings needed to mount your scope.  The larger the objective lens, the higher rings you need to allow your lens to clear the barrel of the scope.  With a typical scope description of 3-9X40, the objective lens is 40mm.

Ocular Lens

The ocular lens is the lens closest to your eye where you actually look into to see the images that are magnified by the scope.  The exit pupil is emitted from this lens.

Parallax

Parallax is the point within the position of the reticle that the target appears at different ranges.  This is most noticeable when the eye is moved off of dead center when looking into the scope.  If you move your eye a little off center to any direction and the crosshair remains on the same position of the target, you are said to be “parallax free”.  If, on the other hand, you move your eye a little bit and the target appears to move within the crosshair, you are experiencing “parallax error”.  Parallax error generally occurs in scopes that are variable powered and have a large range between the bottom and upper end of their magnification. Most rifle scopes are preset to be parallax free at 100 or 150 yards, wheras other scope types such as Rimfire are set at 50 to 60 yards and Shotgun/Muzzleloaders at 60 to 75 yards.  To combat parallax error in variable power scopes, some offer an Adjustible Objective, which allows the shooter to adjust the parallax to given magnification and range.

Power

The power of the scope is the same as magnification. A 10X (ten power) scope magnifies ten times, and makes the object appear ten times larger than with the naked eye.

Resolution

Resolution is the measurement of an optical device’s ability to produce a sharp image by distinguishing fine detail. Resolution is determined by the quality of the glass and coatings, precision manufacturing, atmospheric conditions, and visual acuity of the user.

Reticle

A reticle most commonly known by most people as the “crosshairs” on a scope.  Reticles are the series of lines, circles, dots or other shapes that are viewed in the field of view in the scope to allow a target to be aimed upon.

Trajectory

Trajectory is the flight of your projectile after it leaves the barrel. This flight is an arc. The amount of arc depends on the projectile weight and velocity.

Turret

A turret is one of two knobs in the outside center part of the scope tube. They are marked in increments, most commonly 1/4MOA and are used to adjust elevation and windage for points of impact change. These knobs extend from the turret housing, and are generally either adjustable with your fingers or with a coin.

Twilight Factor

Twilight factor is the measurement of the efficiency of a rifle scope in low light conditions. The higher the twilight factor, the more usable the scope is in twilight conditions. The formula for determining twilight factor is: The square root of magnification times the diameter of the objective lens. Coatings and glass quality are not represented in this figure.

Zero

Zero is the distance that you are sighted in at, and references the flight of the projectile.  This is the process that actually calibrates the trajectory of the bullet in reference where you scope is actually pointing to allow for the combination of the two to meet at the given distance. If you are sighted in at 100 yards, you have a 100 yard zero.  It is common practice to sight your scope in at the distance you most often expect to be shooting at to give the most consistent results