To help narrow the choices, first determine the type of shooting you will be doing. Specialty applications, such as for handguns, shotguns, and forward-mounting “scout” rifles, obviously require purpose-built scopes. Often referred to as “long eye relief” or “extended eye relief,” these scopes deliver a proper image when mounted ahead of the action of a rifle or shotgun or when mounted on a pistol held at arm’s length. Typically of lower magnification (1x to 8x), these scopes are fast-handling and suited to situations where field of view is not a critical factor. Other alternatives to the traditional high-magnification rifle scope include laser sights and red-dot scopes. Laser sights project a red (or green) dot upon the target, but are no more or less “accurate” then any other sighting system and must be adjusted for windage and elevation just like a regular scope. Additionally, they are difficult or impossible to see in daylight conditions, are hard to spot on “soft” non-reflective surfaces, and are typically not allowed for hunting. Red-dot scopes have little or no magnification, and feature a large glowing red dot as an aiming point. This aids the shooter in placing fast shots on running game but masks too much of the target for precise shot placement. (A typical dot will cover four to eight inches of your target at 100 yards.) Some manufacturers use this technology in harmony with their regular scopes, and offer models with an illuminated reticle. This blend produces a fine reticle that sacrifices none of its precision, yet can be made to glow red (or yellow) for use when the target and background are so dark as to prevent accurate use of the black reticle for aiming.

For most “standard” applications, eye relief – the distance from the ocular (rear) lens to the shooter’s eye – becomes an issue of comfort and safety. A generous eye relief is necessary on high-powered rifles whose recoil is likely to drive the rifle rearward enough to cause the scope to contact the shooter’s face. Anyone who has been “kissed” by their rifle scope, and wears the little purple mark on their forehead to prove it, appreciates the benefits of sufficient eye relief – and a firm hold. But for all shooters, the utmost flexibility comes from a scope that offers a longer effective zone of eye relief. A scope that images properly whether your eye is 2” or 2¾” from the objective is easier to mount into existing bases and rings, and provides fast target acquisition during snap shots when your gun isn’t shouldered exactly the way it was when it was sighted in. Typically, the amount of eye relief varies directly with the quality and price of the scope.

A major decision will be the choice of either a fixed or variable power scope (see “How to Read a Scope’s Specifications”). Fixed power scopes excel at shooting conditions where the target is at a constant and pre-determined range, or where the hunting terrain limits the range at which targets may present themselves (close distances in wooded hunting areas, extreme distances in fields of wary varmints). Long-range silhouette competitors need extreme magnification (up to 45x or more) that would be useless on a hunting rifle; hunters may need low power (4x) to find their target fast without sweeping wildly through a panorama of forest, or to place that heart-lung shot without guessing at a lens full of brown fur. A fixed power scope is lighter, less complicated, and more robust.

For shooters under more general conditions, a variable power scope is the answer. Hunters can dial the scope down to the lowest setting of a 3-9x scope for bush hunting, and use the higher settings for fields and cutlines. Target shooters can gain the best view of targets at 100 yards, 300 yards, etc. Even .22 rimfire rifles (typically a short-range cartridge) gain the advantage of low power settings for shots that are even closer than typical, such as that bothersome pigeon in the barn.

One of the most significant optional features of a scope is the adjustable objective lens or side parallax adjustment (see “What is Parallax?”). This allows the scope to be tailor-fit to precise shooting conditions by “telling” it the distance to your target. There are two basic ways to make use of this feature. The shooter can determine the range to the target, either through estimation, a rangefinder reading, or knowledge of the distance (such as at a target range), and dial the distance into the adjustable objective (AO) ring or side parallax knob. Or, he can look through the scope while adjusting the parallax to achieve the clearest focus between the reticle and the target. This will provide an approximate setting. Once proper range information has been input, the parallax will be reduced or eliminated and the scope will deliver its most accurate performance for that distance. Difference in parallax only counts for a very small degree of mismeasure over long distances, and therefore is only of significant concern to competition target shooters and varminters (where the target is very small and absolute precision is required). For a hunter, since only a small window of opportunity exists for taking the shot and the animal’s vital zone is larger, any kind of parallax adjustment simply becomes one more thing you won’t have time to use. For this reason, scopes without AO instead come factory-preset for an “average” distance for typical hunting shots, and will only have measurable parallax error on shots that radically depart from that distance (taking 25 yard or 600 yard shots with your rifle). For this reason, the shooter should only choose the scope meant for his application. Do not select a cheap rifle scope for your rimfire, thinking “it’s just a twenty-two;” find a properly calibrated .22 scope or choose a scope with an adjustable objective.

Another critical feature of a scope is its ability to focus to a particular user’s eye (also known as a diopter setting). In the same way that different people have different eyeglasses (or require none at all), a scope should be adjusted to the individual eye that it is providing an image to. The first step in setting up your scope is to focus it to your eye at its “infinite” focal point. While looking at a non-distinct background at near-infinite distance, such as the sky, turn the focus ring until the reticle becomes a sharp image (some scopes have a locking ring that must be loosened prior to adjustment). Try glancing away from the scope and back again, to allow your eye to “relax” to infinite distance and pick up the reticle without shifting focus. When your eye sees the reticle clearly, the scope is focused for you.

Now that you have selected the basic type of scope that has the right power settings and eye relief for your purpose, and can adjust it for your eye, we can look at other options, features, and differences in quality.

As we mentioned in the sidebar, the diameter of the objective lens is the second critical measurement of a scope after the magnification. Generally speaking, the larger the objective lens, the better capacity it has for transmitting light. (Remember, however, that a scope with a large objective can still waste this capability through poor design or manufacture.) To maintain effective light transmission, special anti-reflection coatings are added to the surface of the glass. Here as well there can be critical differences, such as in the type of substance used in the coating, the precision with which it was applied, and the numbers of coatings and lenses that receive the coatings. Optics (including scopes, binoculars, camera lenses, etc.) that are advertised with “coated” lenses may be far inferior to optics that are “fully coated” or “multi-coated.” In general, a quality scope with a 40mm objective lens will provide adequate light transmission for all normal shooting applications. For the best performance in low-light situations, a quality scope with a 50mm lens may be a better choice. (See “Exit Pupil Limitations” for another interesting factor.) Keep in mind, however, that the larger objective lens (as well as options such as A.O.) requires that the scope be mounted using higher rings, which moves the axis of the scope farther from the axis of the bore and can negatively affect the accuracy of the rifle. Whenever possible, try to select the scope, bases, and rings that will bring the optics as close to the rifle as possible – unless the brightest possible image is truly critical to your type of hunting.

Another often-quoted measurement is for field of view. Generous field of view is likewise provided for by a large-diameter objective, but since optically it is the product of the objective, erector, and ocular lenses, a lesser scope can squander this ability through use of lesser internal components. Measurements for field of view represent the width of the viewable area at a specified distance, usually given in feet (or metres) at 100 yards (or metres).

Reticles (often referred to as crosshairs, though that refers to a specific common style of reticle) come in many configurations. Some companies offer ten or more varieties, plus illuminated versions of the standard offerings. The most commonly encountered reticles include the crosshair (fine cross), duplex (coarse cross with fine centre), and mil dot (duplex type with small milliradian ranging dots). Mil dot reticles can be used very accurately for judging the range to your target (and therefore the hold-over required at that distance), but only if the milliradian system of calculation is learned and understood. Some scopes offer alternatives to the mil dot system for estimating range, such as separate ranging marks displaying specific values or duplex reticles whose fine crosshair centres correspond to a fixed width (such as 16 inches). With such a reticle, if you can find a corresponding width or length on your target (such as the sternum-to-shoulder height of a deer, or the nose-to-tail length of a coyote), you can adjust your magnification to make that assumed length correspond to the known value of the reticle and then read the distance value indicated on the scope’s power ring.

One thing to keep in mind when sighting in your scope is the minute of angle measurement of potential accuracy. The concept of an angle being measured in degrees is common enough; everyone knows that a circle is divided into 360 degrees. Further, each degree is divided into 60 segments called minutes, and each minute into 60 segments called seconds. When a bullet travels toward a target, minor differences in the bullet composition, powder charge, barrel temperature, air density, shooter’s grip on the rifle, and other conditions cause it to impact at a slightly different point each time. The average of those variables can be represented as a “cone” – each bullet should strike within a generally circular zone on the target. That cone-shaped flight path is determined by the degree of variation between shots, and that degree – or fraction of that degree, in minutes – becomes the measurement of accuracy achievable for that rifle with that specific ammunition. In practical terms, one minute corresponds to a width of 1.05 inches (essentially one inch) at 100 yards. Because it is a cone-shaped area of impact, this value translates to two inches at 200 yards, three inches at 300 yards, etc. When a rifle is capable of “half-minute accuracy,” it means that it is consistently placing shot groups that measure half an inch in diameter on a 100-yard target. Many scopes offer windage and elevation adjustments that are graduated in 1-minute, ½-minute, or ¼-minute increments, sometimes with audible “click” detents. When you find that your rifle is shooting two inches high and an inch left (for example) at 100 yards, a scope with ¼-minute adjustments would require four marks or clicks of adjustment downward and two to the right.