This thread is terms that are associated around scopes.
Whether you are new to using scopes or you are veteran riflescope user, these definitions will assist you 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.
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 percentage of light is reflected away to become stray light. This effect is limited when these glass surfaces are treated with a microscopic layer of refractive material, such as magnesium fluoride. To achieve multi-coating, several such refractive layers are applied. When properly applied, coatings can dramatically enhance light transmission properties by delivering more of the available light to the shooter's eye.
Types of coatings include;
Coated - A single layer on at least one lens surface.
Fully Coated - A single layer on all air to glass surfaces.
Multi-Coated - Multiple layers on at least one lens surface.
Fully Multi-Coated - Multiple layers on all air to glass surfaces.
coated vs. fully multi-coated
note - the term air to glass refers to internal and external lenses. Air to glass means that both sides of every lens in side and outside of the scope that contacts air is coated. The term air to glass is used because some lenses are glued together inside the scope and on side of each would not be touching air. Windage and Elevation Adjustments:
Often referred to as adjustments, dials, target knobs or turrets. The windage and elevation adjustments are the dials or knobs used to move the center of the reticle to where the bullet is impacting at a specific yardage. The windage dial is the turret on the side of the scope and is used to move the reticle horizontally (left to right), the elevation dial is the turret on the top of the scope and is used to move the reticle vertically (up and down). Most scopes have a click style adjustment where each click represents a 1/4" movement of the reticle at 100 yards. This adjustment is often referred to as "clicks". Target scopes usually have a more precise 1/8" click adjustments. European scopes mostly use metric increments.Minute of Angle: (M.O.A.)
Defined loosely, one MOA equals 1" at 100 yards, 2" at 200 yards, 3" at 300 yards. If your 5 shot group at 100 yards fits inside a 1" circle then your rifle can be said to shoot 1 MOA. Likewise, if every shot goes into a 2" circle at 200 yards. If you can shoot a 10" group at 500 yards it would be 2 MOA.
There are 360 degrees in a circle. Each degree can be broken down further into minutes. There are 60 minutes in a degree. Likewise, there are 60 seconds in a minute. Now, to figure out the distance subtended by 1 minute at any particular distance, we need merely to plug those two values into a simple trigonometric equation. The tangent function fits the bill nicely. Here's the equation:
tan(angle) = distance subtended/distance to the target
(units must be consistent--e.g., 1/36 of a yard [1"] divided by 100 yards)
Now, we know the angle (1 minute or 1/60 of a degree) and we know the distance to the target (100 yards), but we need to figure out the actual distance subtended at the target (i.e., is 1 MOA actually 1" @ 100 yards?). What we need to do is solve for "distance subtended." Here's our final equation:
tan(angle)*distance to the target = distance subtended
Make sure your calculator is in "degree" mode (as opposed to "radian" or "gradian") and type in 1/60 (for degrees) and hit the "tangent" button. Then multiply that by 100 yards. This should give you the distance (in yards) subtended at 100 yards. Multiply this by 36 to get inches. The answer should be:
This is just a hair over the commonly quoted "one inch." At 1000 yards, this would be almost 10 1/2 inches. It is just a coincidence that 1 MOA happens to be REALLY close to 1" at 100 yards.....quite convenient.Reticle:
The crosshair pattern inside the scope that is used to aim. Reticles can be made of wire or etched onto the glass. The reticle can be placed in the first focal plane (objective plane) like most European scopes. This causes the size of the reticle to change proportionally with the power. When the reticle is in the first plane it always covers the same amount of area on the target because the reticle grows at the same rate that the image does. Very good for low light shots.
If the reticle is placed in the second focal plane (eye piece plane) the reticle will not change when the power is changed but the target will change in size. The reticle will actually take up less area on the target when the power is increases because the target has been magnified but the reticle has not been magnified. Very good for long range shots.More information threads from Skylar@SWFA at below links