Tag Archives: BINOCULARS

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.

SKILLS: Optics ABCs: What All Those Terms Mean

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…in alphabetical order, no less.

Source: NRAFamily.org

rifleman with scope

Contrast
The ability of an optical system to distinguish clearly and crisply between areas of light and dark is called contrast. For shooting purposes, always select the riflescope with the highest contrast.

Exit Pupil
Exit pupil is the diameter, in millimeters, of the beam of focused light transmitted by the ocular lens. The exit pupil can be calculated by dividing the diameter of the objective lens by the power, or magnification, of the scope. An exit pupil of about 5mm or larger in diameter is preferable. A large exit pupil provides a brighter image with greater contrast and a wide field of view for easy target acquisition. Exit pupils smaller than 5mm in diameter offer darker images with lower contrast and progressively narrower fields of view.

Eye Relief
Eye relief is the distance of the eye from the ocular lens when the image fully fills the lens and is not vignetted. Normally, eye relief figures are given as a distance range, for example 3.2 to 3.8 inches, due to differences in individual visual acuity. On a variable-power scope, eye relief typically changes with scope power. Too little eye relief is undesirable, particularly on a scope mounted on a hard-kicking magnum rifle, where it may contribute to a “scope bite” on the eyebrow. For this reason, most centerfire riflescopes have a minimum eye relief of 3 to 4 inches. A riflescope with an eye relief of less than 3 inches should only be used on a small-caliber rifle with low recoil.

Most riflescopes and shotgun scopes are designed to be mounted on the receiver, close to the eye, and thus have relatively short eye relief. Scopes to be mounted on handguns and on the barrels of long guns are classed as long eye relief (LER) or extended eye relief (EER) scopes. Some models provide as much as 18 to 20 inches of eye relief, enabling scope use on a handgun extended at arm’s length. Other models may offer an eye relief of 12 inches or less for scope mounting on a scout rifle. Note that the higher the magnification, the shorter the eye relief of such scopes.

Field of View
Field of view is the width of the area that can be seen in the image at a given distance. Normally, field of view is expressed as the number of feet in the image at 1,000 yards, for example 322 feet at 1,000 yards. Field of view decreases dramatically with increasing magnification. A narrow field of view makes it difficult to find the target and then to hold it in the image. For this reason, a wide field of view may be more important than high scope magnification.

When looking through a scope with a 100-foot field of view at 1,000 yards, a 100-foot-wide object viewed at that distance will just fill the visual field.

Focal Plane
The focal plane is the plane or distance from the objective lens at which light rays from an object converge to form a focused image inside the main tube. Objects in the same focal plane appear to the eye to be at the same distance, and therefore can be seen with equal clarity without the need to refocus the eye. One of the advantages of optical sights is that the target and the reticle are in the same focal plane. This eliminates trying to focus on both iron sights and the target at the same time. This is why riflescopes are so popular with shooters who have less-than-perfect eyesight.

There are two focal planes in a typical riflescope: The first behind the objective lens, and the second behind the erector lens set.

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