Adjustable Objective (AO) - Some Leupold riflescopes have an adjustable objective ring around the objective bell of the scope. This is used primarily by close and long range shooters to bring very near or distant targets into the same focal plane as their reticle in order to eliminate parallax and provide the best possible focus on the target.
Extended Focus Ring (EFR) - An EFR adjustment is a specialized focus ring on a riflescope that is capable of focusing targets at 10 meters and out to infinity.
Adjustments versus Dials - Your Windage and Elevation Adjustments are the actual mechanisms permanently installed in your riflescope to change point of impact. The dial is the part that you physically grip to turn the adjustment. On some models, Dials can be removed, reset, and replaced.
Ballistic Coefficient (BC) - A measurement of how well a bullet overcomes air resistance as it travels. The higher a BC, the less velocity a bullet will lose in flight. The BC of a bullet will typically be published by the manufacturer of that bullet.
Density Altitude (DA) - A measurement of air density relative to standard atmospheric conditions. DA takes multiple factors such as elevation above sea level and temperature, and converts them to a single elevation value for use in ballistic calculations.
Diopter Focus - Most riflescopes will have a focus mechanism on the eyepiece of the scope. This is used solely to bring the reticle into focus for the individual user's eye.
Elevation Adjustment - The dial located on the top of all Leupold riflescopes that is used to change the point of impact of the firearm up or down.
Exit Pupil Diameter - The width of the beam of light between the ocular lens and the eye of the user. In some conditions, a larger exit pupil can provide a brighter image in low light when a high magnification is being used.
Eye Relief - The distance from the eyepiece of an optic where a full image will be presented to your eye.
Eyebox - A common term unused to describe the space your eye can occupy behind a riflescope as still maintain a full field of view.
Field of View (FOV) - The amount visible through an optic at any given moment. FOV can be measured in length at a specific distance (yards @ 100yds) or as an angle (Minutes of Angle). FOV is influenced primarily by the optics length and magnification.
Front (First) Focal Plane Reticles - Reticles located in the Front Focal Plane of a riflescope are located in front of the magnification system. Because of this, when you change magnification you will zoom in and out on the reticle at the same rate as you do the target, allowing any measurements on the reticle to be used accurately at any magnification setting. Front Focal Plane reticles are also commonly called First Focal Plane Reticles.
Handgun (EER) Scope - Leupold scopes described as Handgun or EER (Extended Eye Relief) are designed to have optimal eye relief when mounted to a hand gun held at arm's length. These normally have an eye relief of 16 inches.
Internal Adjustment Range - The amount that a riflescope can adjust the point of impact before running out of travel. This is normally listed as a total-travel number, so a scope that lists 100 MOA elevation adjustment will have 50MOA from mechanical center Up and Down.
Magnification - A measurement of how powerfully an optic magnifies the object being viewed. An optic with a magnification of 1x will present the object at about the same apparent size as the naked eye, while a 10x optic will present the object at about 10 times larger than the naked eye.
Maintube Diameter - The diameter of the maintube of a riflescope where rings would attach. Common sizes include 1", 30mm, 34mm, and 35mm. This should be considered when selecting rings to make sure they are compatible with your riflescope.
Milliradian (Milrad, Mil) - A measurement of angle commonly used by riflescope adjustments to control how much they are moving the firearm's point of impact. One Milliradian is 1/1000th of the distance to the target. For example 1 Mil is 36" at 1000 yds, or 1m at 1000m.
Minute of Angle (MOA) - A measurement of angle commonly used by riflescope adjustments to control how much they are moving the firearm's point of impact. One Minute of Angle is 1/60th of one degree of a 360° circle. One Minute of Angle is roughly 1" at 100yds, 3" at 300yds, etc. More accurately, 1 Minute of Angle is 1.047" at 100yds.
Muzzle Velocity - The speed at which a bullet leaves the muzzle of a firearm, typically measured in Feet per Second (FPS) or Meters per Second (MPS). This is a vital piece of information for use in ballistic calculations.
Objective Lens - The lens of an optic nearest to the object being viewed.
Ocular Lens - The lens of an optic nearest to the eye of the user.
Parallax - An effect in riflescopes where point of impact varies slightly due to a change in position of the shooter's eye behind the scope. Leupold Riflescopes are designed to have minimal parallax for their individual intended purpose. Also, scopes with Adjustable Objective or Side Focus features can change at what distance the optic will be free of parallax.
Power Selector - On variable power riflescopes, this is a dial around the maintube ahead of the eyepiece that is used to change a scope's magnification. Most Power Selectors have a visible screw in them that should not be adjusted by the end user.
Rear (Second) Focal Plane Reticles - Reticles located in the Rear Focal Plane of a riflescope are located behind the magnification system. Because of this, when you change magnification the apparent size of the reticle will not change. This means that if you are using any off-center measurements on the reticle (hold overs or milling marks) you will have to have the scope on a specific magnification. On Leupold riflescopes this is normally the highest power. Rear Focal Plane Reticles are also commonly called Second Focal Plane Reticles.
Reticle - The aiming device of a riflescope. These can have many functions including point-of-aim, rangefinding, bullet drop compensation to name a few. Common examples of Leupold reticles include the Duplex, Boone and Crockett, Target Dot, and Varmint Hunter reticles.
Scout (IER) Scope - Leupold scopes described as Scout or IER (Intermediate Eye Relief) are designed to be mounted on long guns ahead of the receiver (Scout rifles). These normally have an eye relief of 6-9 inches.
Side Focus - Some Leupold riflescopes have a side focus dial on the left side of the turret. This is used primarily by long range shooters to bring distant targets into the same focal plane as their reticle in order to eliminate parallax and provide the best possible focus on the target.
Subtension - The amount of the target that will be covered by some aspect of a reticle. This can be measured in length at a specific distance (inches @ 100yds) or as an anglular measurement in MOA, or MILS.
Turret - The portion of a riflescope's body between the two mounting maintube sections into which are installed windage adjustments, elevation adjustments, and side focus adjustments. Also commonly used to refer to adjustments and dials. (see: Adjustment and Dial).
Twilight Factor - Twilight factor is a metric commonly used in observation products to compare low light performance between models. Twilight Factor assumes that a larger objective lens and higher magnification will aid in low light use. It is calculated as the square root of the product of magnification and objective size in millimeters. √(Mag * Obj). Twilight Factor does not take into account the quality or type of glass and coatings used, so it should not be considered an end-all indicator of low light performance.
Windage Adjustment - The dial located on the right side of all Leupold riflescopes that is used to change the point of impact of the firearm to the right or left.