Telescopic sights have been around since before the Civil War, but only in the years after World War II did manufacturers revisit the design, making improvements to scopes and ultimately offering hunters reliable equipment that would soon become a standard on modern hunting rifles.
What does a scope do for you?
One of the main uses of a riflescope is to magnify your target, giving you a clearer sight picture than with the naked eye. This not only allows you to shoot more accurately at a greater distance, but it also increases safety, since you can better see the target and what lies behind it.
A scope can also give you more hunting time early and late in the day. These are two prime times for big game to be moving to and from bedding areas, but with iron sights, often there is not enough ambient light available to make an ethical shot. Riflescopes gather available light and make it possible for you to accurately shoot in low-light conditions.
A riflescope also allows a higher level of precision than traditional iron sights. At 100 yds., an iron ramp sight will cover up to 6" of the target. Precise bullet placement is limited by the large amount of the target that is covered. However, riflescopes use various reticles (commonly called crosshairs) that in fine target models only cover an 1/8" at 100 yds. This is the ultimate in precision, allowing you to place a shot exactly where you want every time — even in the same hole as the previous one.
Finally, a riflescope allows you to take full advantage of the modern cartridges and rifles available today. New calibers and rifles shoot flatter, farther, and have more energy than turn-of-the-century models. With a correctly mounted and sighted in riflescope and a little practice, hunters can now make precise shots on game at longer distances than before thought possible.
The numbers and what they mean
When you first look at scopes, you will be in awe of the number and models available. Manufacturers use a series of numbers for comparison, and without knowing what they mean, it is hard to make an accurate comparison.
Power
Commonly a riflescope will be expressed in a series of numbers such as 3.5-10x50 or 4x32 (power-power x objective dia.). Power expresses the magnification as a factor compared to the naked eye. So in a fixed power scope, such as the 4x32, the object in view is magnified four times. An object would appear to be four times closer than it would with the naked eye. Therefore, a higher number has a greater magnification. Most scopes sold today are variable power, such as the 3.5-10 mentioned above. This allows greater versatility, since in this case, the shooter can vary the magnification from 3.5 to up to 10, with infinite values in between.
The power that you select depends on the kind of hunting you will be doing. If you are planning on hunting in close cover, you will want either a low-power fixed scope, such as a 4X or a variable that goes down to 3.5X or even lower. This will give you a wider field of view and allow you to acquire a target quickly in close cover. On the other hand, if long range varmint or target shooting is in your plans, you might want a scope that goes as high as 16X or even 20X. This will allow you to see small prairie dogs or the 10-ring clearly at 400 yds. For all around hunting, a range of 3.5-10 or 4-12 will allow some serious range variation, while still dialing down for close shots.
Objective
The second number in a scope, such as the "50" in a 3.5-10x50, is the diameter of the objective lens in millimeters. A larger number indicates a larger lens. Large lenses are more bulky, but they also offer a bit larger field of view and let in more light, which makes your image clearer - especially in low-light conditions.
The low-light performance is due to the maximum exit pupil offered by a larger objective. Exit pupil is the size of the beam of light that leaves the scope. The exit pupil can easily be calculated (in mm) by dividing the diameter of the objective lens by the power. Therefore, a 4x32 scope has an exit pupil of 8mm. On a bright day, the human pupil will vary from 2mm at noon to 4mm later in the day. When your eyes become adapted to dark conditions, such as pre-dawn and after sunset, when big game is moving, the pupil will vary from 5mm to a maximum of 9mm.
On a bright day, having a scope with a larger exit pupil will have little effect. The only difference you may notice is that you will be able to move the scope and still maintain the image. In low light, the exit pupil is the biggest factor in getting as much light as possible to your eye.
Tube Diameter
The majority of the scopes on the market come with the main tube having a 1" dia. Several European models and now a few others also come with a 30mm tube dia. Contrary to popular belief, the larger tube does not allow more light to reach your eye. The exit pupil mentioned above controls this. However, a larger tube diameter does give added strength and rigidity due to the greater cross sectional area and larger rings and mounts. A larger tube diameter also allows for an increased range of adjustment for windage and elevation.
Length and Weight
When carrying your rifle for a long time, every extra ounce can weigh you down. While larger objectives and variable power have their benefits, the extra ounces quickly add up for all these features. If you are looking to minimize the weight of a rifle that you will be carrying a lot, consider a compact, fixed-power scope with a medium-sized objective. It will provide a large exit pupil with a bright image and weigh a lot less than a variable power scope.
How the scope works
Glass
ED glass or extra-low-dispersion glass delivers sharper images and superior color fidelity. The various wavelengths produced by standard lenses are wider and focused at different points along the optical axis. ED glass narrows and directs the wavelengths of visible light into a single focal point, which virtually eliminates chromatic aberrations and produces true-to-life colors.
High-definition glass (HD) is a term that’s often used to refer to ED glass. However, because HD is somewhat understood by the public, it’s also used to describe other optical features. In reality, HD isn’t a type of glass or lens, but a term that describes a higher light transmission or image resolution that goes beyond what the human eye can perceive.
Optical Coatings
The largest limitation of light transmission in riflescopes is reflected light. Anytime that light strikes a glass surface, up to 5% of the light can be reflected back. However, if a thin chemical film (commonly magnesium fluoride) is used to coat the surface of the glass, much of the reflection can be eliminated. The coating reduces light loss and glare, increasing light transmission and resulting in brighter, clearer images. By coating a surface with multiple films, the effect of the coating is increased, at times limiting the amount of reflected light to 0.25% to 0.5% per glass surface.
The reticle is the aiming point within the scope — commonly called "crosshairs" due the standard arrangement being two thin wires that cross. They are also available in different combinations, including pointed posts, dots, multiple dots and bars. One of the more common reticles is a duplex or multiplex design where the main crosshairs are thicker for easier viewing (especially in low light), and as they near the center where they cross, the crosshairs become very fine. This is great for aiming at a background with dense cover. For long-range shooting, you'll want a reticle that displays MIL or MOA marks for hold over. Illuminated reticles vary greatly in their application. For hunting and long range, sub-MOA sized markings are ideal, while red-dots and other illuminated reticles are for close range and fast target acquisition. You will want to choose a reticle that best suits your style of shooting.
Focal Planes
First-focal-plane, or front-focal-plane reticles sit in front of the magnification mechanism, so the reticle scales with magnification for a correct ballistic ladder at any range. It is able to be sighted in at any part of the magnification scale. This also allows a range estimate if you know the target size relative to the MILs on your reticle.
Second focal plane, also known as rear focal plane, is the simplest and most common reticle placement style, sitting behind the magnification mechanism. This means the reticle stays the same size through the entire magnification range. With this focal plane, as you move through the magnification range, the point of impact shifts. This is why the scope needs to be sighted in at maximum magnification, so as you zoom out, the change is almost unnoticeable.
Hybrid reticles are becoming common among scopes with an electronically illuminated aiming point (think ¼-MOA red dot within MIL- or MOA-marked crosshairs). The focal point of the reticle rests in the second focal plane, where it stays the same size. The remainder of the reticle, MIL-spec crosshairs, zooms with your target so the hold-over compensation is accurate through the entire magnification range.
Custom Turrets
Some manufacturers can create custom windage and elevation turrets based on the ballistics you send them. The markings on the new turret typically correspond to the distance shot, to eliminate hold over. Factors include caliber, bullet weight, make, type, ballistic coefficient, velocity, elevation, cold-bore temperature, sight height over the bore line and zero distance.
Parallax Adjustment
The standard focusing knob on all scopes adjusts the reticle to your eye. Parallax occurs when viewing distant targets, and the reticle appears to shift or move. The parallax adjustment puts the reticle on the same focal plane as the target, so when you move your head, the crosshairs stay right where the projectile is going to be. Most scopes without adjustable objectives are factory set to compensate for parallax and focus at an optimum distance of 100 or 150 yds. Parallax is usually only an issue at magnification over 10X. When looking at a scope that will be used for distant targets in higher power, an adjustable objective is a good choice.
Gas Purging
Gas purging aids in the waterproofing process by preventing fogging of the inside of the lenses. Most common are nitrogen and argon gasses with some hybrids of the two in the mix.
Eye Relief
This is the comfortable distance that a scope can be held from the eye and still allow the shooter to see the entire image. It is literally the distance of your shooting eye to the eyepiece. It will usually be stated as a range, since in a variable-power scope the eye relief will vary with the power. Lateral adjustment can be made while mounting the scope to give the individual shooter the optimum eye relief. On a rifle, the more generous the eye relief, the better. This will allow you to acquire the target more quickly, which is a must for running shots. 3" to 4" is a good number that will fit most hunters. On large, magnum rifles, you want a maximum of relief, so when the rifle recoils, the scope doesn’t come back and possibly injure the shooter.
Common terms to understand
Field of View
Or FOV for short. What this means is how wide the area is (in ft.) that you can view at 100 yds. A higher number indicates a wider area, while a smaller number indicates a narrower area. The focal length of the objective lenses and the eyepiece design have the most impact on the actual FOV. The power of the scope has an inverse relationship with FOV. As the magnification increases, a smaller FOV results. If you are looking for a scope for quick target acquisition in close cover, you will need a wider field of view and therefore, a smaller power.
Minute of Angle
Or MOA for short, is a term to designate variances on a target at 100 yds. distant. Most commonly, it is used to describe the adjustment on a scope. 1 MOA is equal to 1.047". If a scope’s adjustments are listed at 1/4 MOA, then for every click of the adjustment knob, the bullet’s point of impact will move 0.26175", rounded to 1/4" at 100 yds.
MILs
Short-hand for milliradians, serves the same purpose as MOA; however, the math behind them is vastly different. MILs are more suited for fast target acquisition and quick shooting because there is more hold over involved than MOA. Here is a quick math lesson on MILs and MOA. 1 MIL = 3.438 MOA, 1 MOA = 1.047", both at 100 yds. So to find out how many inches are in 1 MIL, we have to multiply how many inches are in 1 MOA times how many MOA are in 1 MIL. The equation is 1.047 x 3.438 = 3.599" per MIL (rounded to 3-2/3"). If the scope says it is adjustable in 0.1 MIL clicks, this translates to 0.3599 MOA at 100 yds., or 0.3768153", or rounded to 1/3" for simplicity.
Windage and Elevation
Windage is the term for horizontal adjustment of your scope. Elevation is the adjustment of the scope in the vertical direction.
Types of scopes
Air-gun Scopes
The unique recoil pattern of spring piston air guns requires the purchase of a special air-gun scope. Unlike centerfire and rimfire rifles that recoil only in one direction, airguns recoil both rearward and forward. This double recoil action can damage scopes that are not designed to handle it.
Crossbow Scopes
Like airguns, crossbows also boast unique recoil patterns that can cause regular rifle scopes to lose their zero. Also, the reticles with drop compensation accommodate a bolt's flight trajectory.
Handgun, Shotgun and Muzzleloader Scopes
In short, they're all built to take on the recoil associated with the type of firearm they’re made for.
When mounting a scope on a handgun, you will need a special pistol scope that has an eye relief of 12" to 24". This will allow you to hold your handgun in a comfortable shooting stance and still see the full image through the scope.
Shotgun and muzzleloader scopes are configured to hunt in close quarters, 200 yds. or less. Their powers are low, with a max of 9X. Objective lenses are small to fit over the large barrels.
Purpose-Specific Configurations
Built for predator hunting and tactical use, nightvision and thermal-imaging scopes and accessories allow hunters to take safe, calculated shots without shining spotlights on the animals, preventing spooking.
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