Tag Archives: RIFLESCOPES

SKILLS: Riflescopes: Adjustments & Variable Power

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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

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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: Problems (Some) Riflescopes (Can) Have

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The more you know the better choices you can make. Consider all of this carefully before you purchase your next riflescope.

by NRA Staff

There are some problems that riflescopes can experience, but you should note that modern manufacturing techniques can make a real difference. There are three main issues:

Parallax
Many riflescopes suffer from a condition that stems from the inability of a scope to remain focused at all ranges. The compromise solution for most scopes is to design them to focus at infinity or one specific range. This serves most purposes and simplifies scope design. When a scope is properly focused at the chosen zero range, parallax will be minimal.

However, this is not acceptable for some applications, such as varmint shooting and hunting at long ranges. Under such conditions, parallax becomes a problem that must be addressed. Scope makers solve this problem by offering models with adjustable objective (AO) lenses. AO models incorporate adjustable objective bell housings with graduations marked on the traveling edge that allow quick and easy adjustment to remove parallax at any range. Alternately, some models locate the parallax adjustment in a third turret on the main tube for more convenience. Although AO and side-focus models cost more, shooters demanding enhanced accuracy often feel they are worth the asking price.

Sealing
Most quality scopes are sealed. This means the outer lenses and adjustment systems must be sealed against ingress of water, dust and dirt. This is very important, as dust or dirt inside the tube will degrade the image in several ways, mainly by appearing as black spots within the field of view. Dirt inside the tube can also jam the delicate adjustment system. Moisture inside the tube can cause fogging so that the shooter cannot see through it. Moisture can also cause corrosion of inner parts and surfaces.

Scopes are sealed at the factory by first attaching them to a vacuum pump that removes all air from inside the tube. The tube is then filled with dry nitrogen gas to prevent fogging and then subsequently sealed. Of course, if you remove a turret or the ocular bell housing, the nitrogen gas may escape, thus compromising your scope’s anti-fogging capability.

Many high-quality scopes have double seals to ensure gas-tight integrity. However, no scope is permanently waterproof despite advertising claims to the contrary. Wear, tear, impacts and age all conspire against the tube holding the nitrogen gas. For this reason, most scope manufacturers will reseal and refill a scope at modest cost.

Want to check your scope for leaks? Try this simple test: Fill a sink or washbasin with warm water. Immerse your scope in the water for five minutes and check for bubbles coming from the tube. Bubbles mean leakage and such scopes should be sent back to the manufacturer for resealing and refilling.

Shock & Recoil
Newtonian physics are not kind to riflescopes. In addition to maintaining their accuracy, reliability, and water-tight integrity, scopes must withstand the considerable shock of repeated recoil many times the force of gravity. The delicate adjustment mechanisms and lens mounts are particularly susceptible to high G loads and must be designed accordingly. Scope makers are well aware of this and have designed shock resistance into their products. They have been so successful that shock resistance is now taken for granted by shooters and manufacturers alike.

Air rifles are a special case. Be careful when using conventional riflescopes on a spring-piston air rifle. If you do, the lenses may come loose, sometimes within a few shots, and your scope could be damaged or ruined. The reason is that spring-piston air rifles recoil in both rearward and then forward directions while a conventional rifle recoils only rearward. Thus, a riflescope for a conventional firearm need resist G forces in only one direction — rearward. Air rifle scopes must resist G forces in both directions. This requires a special scope designed for the purpose.

SKILLS: Riflescopes: Lens Coatings

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Lens coatings provide superior optical clarlty and utility. Here’s how and why…

coated lenses

Source: NRAFamily.org

In any optical system, some light is lost through reflection each time the light passes through a glass-to-air surface. The light loss can be significant in multi-element riflescopes; as much as 50 percent of the light may be lost to reflection as it passes through an uncoated lens system.

In the 1940s, it was discovered that magnesium flouride coatings on lenses would increase light transmission, color fidelity and image brightness considerably. Today, nearly all modern scopes have coated lenses that transmit from 95 to 99 percent of the light that enters the objective lens.

Coatings such as zinc sulfide and zirconium oxide are used, often in combination with magnesium flouride. A coated lens will appear tinted when viewed from the side. The exact color may vary from blue, green, purple, red or gold. Abrasion-resistant coatings have been developed for the exterior lens surfaces of modern riflescopes. Water-shedding coatings have also been developed.

Various levels of coating can be applied to lenses ranging from a single layer of magnesium flouride on the exterior objective and ocular surfaces, to as many as 15 layers or more on every surface of every lens. Typically, coating layers are only a few ten-thousandths of an inch thick.

The term “fully coated” when applied to a riflescope usually means that all lens-to-air surfaces have at least one coating layer. This includes the interior lens systems as well as the exterior.
The term “multi-coated” or “multiple-layer coated” signifies that multiple coating layers have been applied to some, but not all, lens surfaces. Normally, this means that only the outer lens surfaces have been multi-coated. “Fully multi-coated” signifies multiple coatings on all lens-to-air surfaces.

Lower-priced scopes may have from one to five lens-coating layers while more expensive scopes may have as many as 15 or even more. In lower-priced scopes, only the outside surface of the objective (front) and ocular (rear) lenses are coated. Higher-quality scopes have all internal and external lens surfaces multi-coated.

How many layers are enough? That depends on the quality of the lens system and the intended purpose of the scope. Adding more layers of coating rapidly reaches the point of diminishing returns, but on a high-quality scope where maximum light transmission and image fidelity are necessary, 15 layers of coating can be easily justified.

SKILLS: Riflescopes: All About Reticles

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This is the second in a series on optic basics, and it covers the most visible component of a scope: the reticle. Read on…

by NRA Staff

basic riflescope reticle

The riflescope’s reticle is the visible reference used as an aiming point to align the gun with the target. There are many reticle patterns ranging from simple to complex. The most popular remains the general-purpose crosshair. However, even the simple crosshair offers choices, such as tapered, ultra thin, duplex, mil-dot, ballistic compensating, range-finding, center dot, center ring and post, just to name a few. Each configuration is intended for a specific type of use and there are multiple versions of all. For example, tapered crosshairs are a popular choice for varmint hunting and duplex crosshairs are a common choice for big-game hunting. There seems to be no limit to the new reticle designs being offered, and most makers offer at least six or more types. Your best bet is to try out several at a local gun store, then consult with experienced shooters or hunters before making a final selection.

Reticles may be illuminated electronically, with tritium or with fiber optics to enhance their contrast against dark backgrounds; this is helpful especially at dusk or dawn or during heavy overcast conditions. Illumination remains an expensive option that may not work well in very cold conditions and has limited usefulness. Still, it has proven a popular addition to many scopes.

reticle choices

The reticle itself may be located inside the scope at the first, or front, focal plane or the second, or rear focal plane. The location is an issue only in variable-power scopes. Reticles located in the first focal plane in a variable-power scope will increase or decrease in size as the magnification is changed while those located in the second focal plane do not change size when the power is adjusted. For this reason, the latter location has become the most popular.

One situation in which a front-focal-plane reticle is clearly advantageous is in scopes with a mil-dot ranging system. This type of reticle employs dots spaced one milliradian apart on the crosshair. (A milliradian is the angle subtended by 3 feet at 1,000 yards.) An object of known size is bracketed between the dots, and a table is used to determine the range based on the number of dots the object measures. With a rear-focal-plane reticle variable, the mil-dot system is only accurate at one power setting. A front-focal-plane location maintains the same relationship to the target throughout the range of magnification, thus enabling mil-dots to be used accurately at any power.

A second benefit of placement in front of the variable-magnification lens system is that the reticle remains unaffected by tolerances or misalignment of the erector tube during power changes. With a rear-focal-plane location, these tolerances may shift point of impact as the power level changes.

In the past, many scope reticles were not constantly centered, meaning they moved off to the side when windage or elevation adjustments were made. Many shooters found this annoying. Today, nearly all riflescopes have constantly centered reticles that do not change position when adjustments are made.

Crosshairs or other reticle patterns are created by laser etching on optical glass or by ultra-thin platinum wires. Some early scope reticles used strands of hair, hence the name “crosshairs.” Others used spider silk…interestingly enough, the silk of the black widow spider, which has a better tensile strength than other types of spider silk.

David Tubb DTR Reticle
David Tubb’s DTR design takes a reticle about as far as it can go, offering built-in aiming dot compensation choices to even account for density altitude.

SKILLS: Optics ABCs: What All Those Terms Mean

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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.

See the huge selection of riflescopes available here at Midsouth HERE