RifleScope School

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Rifle scope Terms

Whether you are a novice or seasoned riflescope user, these definitions will assist in understanding the different components and considerations that go into the engineering of riflescopes.

Magnification 
The most basic characteristic of a given scope, magnification is simply a measure of how many times better you can see an object than with your naked eye. It's typically the first number you see in a scope's name. For example, the a 6x42  has a six-power magnification: Six times better than the naked eye.
Variable power scopes allow a range of magnification for different hunting situations. In these scopes, the first two numbers, separated by a hyphen, indicate the range of magnification. For example, a 3-9x36 magnification can be adjusted as low as 3x, or as high as 9x.

Objective Lens Diameter  
The second figure in the specification, e.g. 6x42 on a fixed power scope have 6x magnification with a 42mm diameter objective lens.  A variable scope's specification would look like this, e.g. 3-9x36.  3x-9x with a 36mm diameter objective lens.  The number indicates the entrance aperture diameter or entrance pupil in mm.  It is a measure of the amount of light that can enter a scope.  Keep in mind that effective objective differs from outside diameter.

Ocular Lens
The Lens closest to your eye.

Field of View
This important term describes the actual width of your scope's sight picture at a specific distance.  Field-of-view is a function of magnification and the focal lengths of the objective and eyepiece lenses. But remember this: The higher the magnification, the narrower the field-of-view. Field of view is determined by the ocular lenses in the eye piece.  Different factories and brands will vary in design and stats.  Decreasing the eye relief in a scope will widen the field of view.  So its a trade off one way or the other.

Exit Pupil
The size of the column of light that leaves the eyepiece of a scope (usually measured in millimeters).  The larger the exit pupil, the brighter the image.  To determine the size of the exit pupil, divide the objective lens diameter by the power of the scope.  IE; a 4x32 scope would have a 8mm exit pupil.  32/4=8.

Twilight Performance  
Using the following formulas gives a basic evaluation of low light performance; however, one must keep in mind that they are mathematical formulas and do not take into effect some of the most critical features in optics: glass quality, number of lenses, precision of manufacturing and coatings.

Eye Relief  
This describes the distance between your shooting eye and eyepiece lens. It's an important safety consideration. Because if the eye relief is too short, there's an increased risk of dangerous contact between you and your scope under recoil.  Eye relief is determined by the field-of-view, and by the focal lengths of the objective lens and eyepiece lens. Generally, the higher the magnification and the larger the field-of-view, the shorter the eye relief.  Some scopes offer a soft neoprene eyepiece guard in case you get too close.  The Swarovski Professional Hunter line of scopes all have a recoiling eyepiece in addition to the soft neoprene rubber guard for the ultimate in "scope eye" protection.

Parallax 
Parallax is essentially an optical illusion.  Parallax presents itself as the apparent movement of the reticle, in relation to the target, when your eye moves off center of the sight picture (exit pupil) or in more extreme cases it appears as an out of focus image. It indicates that the scope is either out of focus or more specifically the image of the target is not occurring on the same focal plane as the reticle. Maximum parallax occurs when your eye is at the very edge of the sight picture (exit pupil). Even when parallax is adjusted for a designated distance, there is an inadvertent error at other distances.  Most brands of scopes that do not have a parallax adjustment are pre-set at the factory to be parallax free at or around 100 yards; rim fire and shotgun scopes are set at or around 50 yards.  Most scopes of 11x or more have a parallax adjustment because parallax worsens at higher magnifications.  Generally speaking parallax adjustment is not required for hunting situations and is primarily a feature used and desired by target shooters.  A 4x hunting scope focused for 150 yards has a maximum error of only 8/10ths of an inch at 500 yards.  At short distances, the parallax effect does not affect accuracy. Using the same 4x scope at 100 yards, the maximum error is less than 2/10ths of an inch. It is also good to remember that, as long you are sighting straight through the middle of the scope, or close to it, parallax will have virtually no effect on accuracy in a hunting situation.

Center Tube Diameter (1", 30mm, 26mm)  
The diameter of a scope's center tube (or main tube) impacts the overall strength and durability of the scope. And it obviously determines the size of bases and rings required for mounting. But beyond that, the center tube diameter must be adequate to allow a sufficient range of windage and elevation adjustment.

Stray Light
When light entering the scope reflects off of air-to-glass surfaces, the reflected light eventually exits in the scope in the form of stray light. This unfocused light typically diminishes the image quality of the sight picture.  To limit the detrimental effects of stray light, manufacturers employ proprietary lens coatings. Additionally, all interior surfaces can be anodized in a matte black finish to prevent reflection of the metal. Also, some higher end scopes do not contain any lubricants, such as oil, that may leak inside and reflect light. The net result of these manufacturing techniques create an image that is crisp and true to color.

Lens Coatings
As light strikes an air-to-glass surface, a small 