Optics Terms and Information
Terms:
Adjustable Objective
An adjustable objective is a dial around the objective end of the scope, or a knob on the left side of the turret housing that allows 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.
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Airgun Scope:
Airgun scopes are specially designed to withstand the dual-recoil of spring piston airguns. Powerful spring piston airgun recoil will destroy non-airgun rated scopes in short order. Airgun scopes are often sealed on both sides of the lenses and made more strongly overall. They also have adjustable objectives to allow the shooter to use high magnification at short distances. Official airgun distances are 10 meters (33 feet), 25 feet, and 50 feet.
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Coatings:
Microscopic coatings on the lens surfaces reduce light loss and glare due to reflection. Coated lenses offer a brighter, higher contrast image with less eyestrain. More coatings allow better light transmission, but it is very possible to have a scope with a single coating to greatly outperform a scope with multicoated lenses. It all depends on the quality of the coatings as well as the glass. Good quality does not come cheap. The following are accepted terms for coatings:
* Coated: A single layer on at least one lens surface.
* Fully Coated: A single layer on all air to glass surfaces.
* Multicoated: Multiple layers on at least one lens surface.
* Fully Multicoated: Multiple layers on all air to glass surfaces.
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Click:
A click is one adjustment notch on the windage or elevation turret of a scope. One click most often changes a scope's point of impact 1/4 inch at 100 yards. Some clicks are 1/8 inch, 1/2 inch, one inch, or even more.
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Exit Pupil:
An exit pupil is the small circle (column) of light that is visible in the ocular lens when you hold your scope (or binocular) at arm's length. The larger the exit pupil is, the brighter the image that will be entering your eye. To find the exit pupil for your scope, divide the objective lens diameter in millimeters by the magnification. For example, if your scope is four power (4X), and your objective lens is thirty-six millimeters in diameter (36mm), divide 4 into 36 and it equals 9. Nine would be the exit pupil size in diameter in millimeters.
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Eye Relief:
Eye relief is the distance your eye must be from the ocular lens and still get a full field of view.
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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.
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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.
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Kentucky Windage:
Kentucky windage is the amount of point of aim change either left or right of your target, without adjusting your scope, to adjust for wind effects on your projectile.
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Magnification:
Magnification is the power rating of the scope, indicated by the symbol "X". A 10X (ten power) scope makes objects seem ten times closer than with the naked eye.
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Maximum Point Blank Range:
The longest distance you can hold dead center in your kill zone and not be too high or too low for a hit in the vital area. Flatter trajectories and/or larger targets increase this range.
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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, etcetera.
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Objective Lens:
The objective lens is the lens closest to the object being viewed. It is measured in millimeters in diameter. A larger objective lens allows more light to enter the scope. In a three to nine by forty (3-9X40) scope, the 40 is the objective lens size.
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Ocular Lens:
The ocular lens is the lens closest to your eye.
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Parallax:
Parallax is the apparent position of the reticle on the target image at different ranges. This is most visible when the eye is moved outside the center of a scope while viewing a target at other than the standard distance at which the scope has been preset to be parallax free. Most riflescopes without adjustable objectives are set at 100 or 150 yards. Rimfire scopes are often set at 50 or 60 yards, and shotgun scopes are often set at 60 or 75 yards.
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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 closer than with the naked eye.
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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.
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Reticle:
A reticle is a system of lines, dots, or crosshairs in your scope that appear superimposed on your target.
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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.
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Turret:
A turret is one of two knobs in the outside center part of the scope tube. They are marked in increments, and are used to adjust elevation and windage for points of impact change. These knobs protrude from the turret housing.
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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 useable 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.
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Zero:
Zero is the distance that you are sighted in at, and references the flight of the projectile. If you are sighted in at 200 yards, you have a 200 yard zero.
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Reticle focal plane:
Typical internal construction of a scope with its reticle in the First Focal Plane.
The reticle may be located at the front or rear focal plane (First Focal Plane (FFP) or Second Focal Plane (SFP)) of the telescopic sight. On fixed power telescopic sights there is no significant difference, but on variable power telescopic sights the front plane reticle remains at a constant size compared to the target, while rear plane reticles remain a constant size to the user as the target image grows and shrinks. Front focal plane reticles are slightly more durable, but most American users prefer that the reticle remains constant as the image changes size, so nearly all modern American variable power telescopic sights are rear focal plane designs. European high end optics manufacturers often leave the customer the choice between a FFP or SFP mounted reticle.
Variable power telescopic sights with front focal plane reticles have no problems with point of impact shifts. Variable power telescopic sights with rear focal plane reticles can have slight point of impact shifts through their magnification range caused by the positioning of the reticle in the mechanical zoom mechanism in the rear part of the telescopic sight. Normally these impact shifts are insignificant but make accuracy oriented users, that wish to use their telescopic sight trouble-free at several magnification levels, often opt for front focal plane reticles. Around the year 2005 Zeiss[9] was the first high end European telescopic sight manufacturer who brought out variable magnification military grade telescopic sight models with rear focal plane mounted reticles. They get around impermissible impact shifts for these sights by laboriously hand adjusting every military grade telescopic sight. The American high end telescopic sight manufacturer U.S. Optics Inc.[10] also offers variable magnification military grade telescopic sight models with rear focal plane mounted reticles.
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