Tag Archives: OPTICS 101

SKILLS: Riflescopes: Adjustments & Variable Power

There’s a lot to learn to really understand riflescopes and make the best choice. Here’s another valuable lesson. Keep reading…

riflecope

SOURCE: NRA Staff

Windage and elevation adjustments in riflescopes are made with either internal or external adjustment systems. Here’s what that means.

Internal: Most modern telescopic sights have internal adjustment systems using threaded, cylindrical knobs or screws in the turrets. The adjustment screws move the reticle assembly in the optical axis inside the main tube against spring pressure. The adjustment screws have clearly marked graduations around their circumference and many have a ball-detent system that clicks as the adjustment screws are turned. Each graduation or click represents a change in reticle position that moves the bullet strike at the target. This is expressed in minutes of angle (m.o.a.) and normally has a value of 1/2, 1/4 or 1/8 m.o.a. per click.

External: Many older scopes have an external-adjustment system built into the mounts and rings. Such scopes remain popular today for certain types of target competition. In this type of scope, the reticle remains stationary within the main tube and the point of the bullet strike is adjusted by mounts having micrometer windage and elevation mechanisms that move the entire scope laterally and/or vertically. These mounts often allow the scope to slide fore and aft to reduce recoil. An advantage of external-adjustment scopes is that the user is always sighting through the optical center of the tube.

As internal-adjustment systems became more reliable and more accurate, the popularity of external-adjustment scopes faded. Today, external-adjustment models are still offered, however the use of such scopes is now generally limited to a few specialized disciplines of rifle competition.

It is important to note that some scope-mounting systems designed for internal-adjustment scopes still incorporate the ability to accommodate some coarse external windage adjustment.

This leads us to the discussion of variable power. Variable-power riflescopes have an internal mechanism to change the amount of magnification within design limits. This consists of an additional set of lenses mounted in an internal tube that slides forward and rearward under the control of a cam attached to the magnification ring. The design of the lens system and its position in the tube controls the amount of magnification.

The popularity of variable-power riflescopes rests squarely on their flexibility. Variable magnification enables the shooter to adjsut the power to suit a wide variety of conditions ranging from lower power (with a wide field of view for fast shots at close range), to higher power (for greater precision at long range). Once considered expensive and unreliable, variable-power riflescopes have become the most popular type as their design has matured and prices have dropped. Todya, the single most popular riflescope is the 3-9X-40mm, which has become a kind of “jack of all trades.” Smaller variables such as 2-7X-32mm remain popular for smaller-caliber rifles, while 4.5-12X-50mm and bigger models are favored for long-range shooting. Despite their flexibility, no one variable fits all applications and that is why there are so many different models.

Despite their popularity, variable-power riflescopes may suffer from certain drawbacks:
ONE: The variable magnification system introduces another level of mechanical complexity and another source for optical error, potentially decreasing reliability.

TWO: The movement of the internal components of a variable-power scope can produce changes in zero as the scope power is increased or decreased.

THREE: Variable-power scopes are harder to seal than fixed-power scopes by virtue of the magnification-adjustment ring.
As the magnification increases, the field of view and image brightness decrease, often substantially.

FOUR: Variable-magnification scopes are substantially heavier than fixed-power scopes.

FIVE: Variable-power scopes are more expensive than fixed-power scopes.

Optics Terms Defined: Magnification and Objective Lens

When it comes to optics for firearms, the specific terms that people use to describe them can be confusing. Here’s what all that argot actually means…

optics array

by NRA Staff
SOURCE: NRAFamily

Magnification
The magnification, or power, of a riflescope is expressed as a number corresponding to the size of an object viewed at a specified distance through the scope, relative to its size as seen with the naked eye. Put another way, an object 100 yards distant viewed through a 10X scope will appear to be the same size as if it were viewed with the naked eye from 10 yards away. Different scope magnifications are used for specific shooting activities.

High-magnification riflescopes from 15X to 50X with objective lens diameters of 40-50mm or more with adjustable objective-lens systems are popular for various types of centerfire rifle competitions such as benchrest and F-Class.

Varmint shooters normally prefer a scope with magnification levels of 12X to 24X and adjustable objective lens diameters of 44-50mm for their precision work.

Long-range big-game hunting demands a scope with an adjustable objective lens system of approximately 40mm diameter with power levels up to 15X that enable the hunter to judge game and wind conditions at extreme distances.

At dawn, dusk or during poor light conditions, scopes with large objective lenses of 50mm and above that gather all existing light are preferable, with powers between 6X and 12X. Illuminated reticles are a popular option on these scopes.

Low-power scopes of 1.1X to 4X with a wide field of view and fixed objective are well-suited for hunting in woods or brush at close range.

For general-purpose hunting, most sportsmen are well served by a 3-9X-40mm variable scope with fixed objective, which is a good compromise between a wide field of view for close shots (at 3X) and added magnification (at 9X) for distant shots.

As magnification levels increase, the field of view decreases, which makes target acquisition increasingly difficult. Increasing magnification also magnifies movement, making the reticle appear less steady and thus hampering the ability of many shooters to hold their point of aim. These factors conspire to make most scopes over 8X very difficult to use without a solid rest. When shooting from a rest on a bench, a narrow field of view and high magnification are less of a problem.

Objective Lens
The objective lens is the light-gathering lens at the front of the scope. The larger the diameter of the objective lens, the more light will be admitted into the scope. This results in a larger exit pupil with a brighter image.

Most riflescopes have objective lens diameters from 32mm to 44mm. These provide a good balance between light-gathering capability, cost and image quality. Such riflescopes are relatively lightweight and easy to mount on most rifles. For many hunting applications, such riflescopes are an excellent choice.

For hunting at dusk, dawn or in very low light conditions, the increased light-gathering capability of a larger objective lens may be a better choice. For such conditions, most scope manufacturers offer models with 50mm to 56mm objective lenses. However, there is a penalty to be paid for th is increased performance in the form of substantially increased weight, higher cost and difficulty in mounting a scope with such a large objective.

Varmint hunters and some target shooters prefer riflescopes with large 50mm or greater objective lenses for a different reason. They want a higher-power scope of 12X or more with a clear, crisp, flat image with excellent contrast and an adjustable objective to remove parallax. The image quality reduces eyestrain and enables them to clearly see small targets at long ranges and to judge wind and mirage precisely. They also spend considerable time looking through the scope with the rifle held on a solid rest, so unsteadiness from high magnification and a narrow field of view is less important.