Gun Scopes Terms

Gun Scopes

Gun Scopes – Before you can determine what type of gun scopes you really need, you should understand the following terms for gun scopes.

Exit Pupil:  The exit pupil on gun scopes is the small circle of light that appears in the eyepiece.  The formula for determining the exit pupil size is the objective lens diameter size in millimeters divided by the magnification.  The larger the exit pupil on the gun scopes, the less critical the position of your head is in relation to the gun scopes.  Also keep in mind, that an older person’s eyes may only dilate to about four millimeters in low light situations while the younger person’s eyes may open up to seven millimeters or more. Therefore, a larger exit pupil size on gun scopes will become less of a deciding factor for the older eye.

Eye Relief:  The eye relief on gun scopes is the distance that your eye must be from the eyepiece on gun scopes to get a full, clear picture.  Lower powered gun scopes will have a larger range of eye relief, while higher powered gun scopes have a more critical range for positioning your eye from the eyepiece.  It is also important to take into consideration the recoil from the gun.  The greater the gun’s recoil, the more concern for available eye relief is needed to avoid injury when the gun scopes rebound and can potentially strike your eye.

Lens Coating:  See anti reflection coating.

Magnification:  The magnification on gun scopes is the measure of how many times larger an object will appear than with your naked eye.  Gun scopes can also come in variable or adjustable powers that offers a range of magnifications.  This information is indentified by a range of numbers that are separated by a hyphen on gun scopes.  Beware of the notion that “the more magnification – the better.”  In reality, the more magnification you have with gun scopes, the less light that gets to your eyepiece thus reducing the image brightness, contrast, and clarity, while also reducing the gun scopes field of view.

Field of View (FOV):  The FOV on gun scopes is measured in feet at 100 yards.  This is the amount of view you see through the gun scopes from right to left at that distance.  FOV is inversely effected by magnification – as magnification increases, the smaller the FOV, and conversely, as the magnification decreases, the greater the FOV.  For example, lower power gun scopes provide a better configuration if you will need to be acquiring targets at a close range.

Objective Lens:  The objective lens diameter on gun scopes is measured in millimeters (mm) and generally range in size from 32 to 75mm.  The larger the objective lens, the brighter the target image and the higher the contrast and clarity of these gun scopes. However, the negative side to a larger objective lens is that the bigger glass elements are heavier in weight.  For gun scopes with objective lenses greater than 48mm , they typically require special mounting rings and bases mounting.  These mountings will likely be positioned higher off the rifle which will impact eye relief resulting in a less comfortable shooting position.

Parallax Adjustments:  The most misunderstood and often incorrectly made adjustment on rifle scopes.  The purpose of the parallax adjustment is to eliminate the unwanted optical error that makes the crosshairs appear to “move” when shooting at different distances through rifle scopes.  This adjustment usually needs to be set more accurately (with the rifle securely positioned on a bench) so that you can look through the gun scopes while shifting your eye to the left and right so that you can see if the crosshairs appear to move across your target.  Low powered gun scopes have a fixed parallax setting that is exactly correct for shooting at only “one” specific distance.  Rifle scopes with a higher magnification require a parallax adjustment that can be adjusted for shooting at any distance, so that your crosshairs will appear to be rock solid, no matter where your eye is positioned.  This is very important when the “exact” position of your eye is not always concentric with the gun scopes and the slightest variance can make a huge difference.  On some rifle scopes, it will be a good idea to re-label the parallax yardage markings, so they can be positioned according to the “actual” yardage.

Reticle:  The pattern placed in the eyepiece of the gun scopes which helps to establish the gun’s position on the target.  Reticles for gun scopes can be made from fine wire or etched into the glass itself and come in a wide variety of patterns that range from simple traditional style crosshairs all the way to illuminated mil dots (short for military dot).  Some manufacturers of gun scopes produce their own specialized reticle.  Make sure that you choose a reticle for that fits to your specific type of shooting.  For example, long range shooting, hunting or competitive shooting, you should consider gun scopes that utilize a reticle without a dot that could potentially obscure your target.

Turret Adjustments and Minute of Angle (MOA):  Target turrets are designed to fine tune the sighting of gun scopes for rifles.  Turrets are housed in the center of the gun scopes tube in a protrusion called the turret housing and have knobs on the top and side of the gun scope which allow you to adjust the elevation (up / down) and windage (left / right) on the aiming reticle.  There are two main styles of turrets: Hunting and Target.  Hunting turrets are the more traditional style and have a lower profile and usually have covers on them for protection that you remove to make adjustments.  Hunting turrets offer good protection to the adjustment mechanism from snags and bumps but making adjustments is slower and usually requires a coin or screwdriver to turn the mechanism.  Since Target turrets are tall, their use is most suited to where they will not get banged around or snagged.  This is why you will not usually find Target turrets on gun scopes where a much lower profile is desirable.  Turret adjustments are in “Minutes of Angle” (MOA) which is a unit of measurement of a circle.  For all practical purposes, it is equivalent to 1 inch at 100 yards.  The adjustments on these gun scopes are most often made in 1/4 inch increments at 100 yards, with each “click” of an elevation or windage turret, moving your point of impact by 1/4 inch at 100 yards.  For example, this change translates to moving your impact point by 1/8 inch at 50 yards or 1/2 inch at 200 yards.  So if your bullet hole is four inches low at 100 yards and you have a scope with 1/4 minute clicks, you will need to adjust your dial in the direction of the “up” arrow on your gun scopes turret by 16 clicks. There are some gun scopes that have clicks that are 1/2 inch or even 1 inch while other gun scopes have adjustment dials that do not have clicks at all with a friction-type adjustment that is infinitely adjustable.

Bullet Drop Compensators (BDCs) – Bullet drop compensators for gun scopes for rifles are turret adjustments matched to your caliber and bullet weight.  Most bullet drop compensators are matched to common military calibers and weights.  They work by having you estimate the range to your target and then adjusting the gun scopes turret to the corresponding distance marked on it.  Even if a BDC was made for a particular load from a particular gun, it still would not be a perfect adjustment due to such variables as temperature, elevation, humidity, barrel cleanliness, and lot number of ammo.  Minor changes will make big differences, and the further the distance the further off your shot can be. So for most guns, most BDCs can be used only to get you close to your target with your gun scopes.


This entry was posted in Gun Scopes. Bookmark the permalink.

Leave a Reply