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Riflescope Guide

Riflescope Terminology

The brilliance and sharpness of the image you see through a particular riflescope is determined by a number of different factors, including the combination of these factors. Magnification, optical coatings and lens diameter are just a few of the factors influencing how a riflescope performs.

However, the single most important performance feature will always be the quality of the optics.

Please consider the following factors when choosing a riflescope:

Magnification (Power)

Diameter of Objective Lenses

Field of View

Exit Pupil

Eye Relief

Parallax

Center Tube Diameter

Length

Weight

Straylight

Lens Coatings

Construction

 

 


Magnification (Power)

Magnification is the degree to which the object being viewed is enlarged. For example, with an 8x42 riflescope, the number 8 represents the riflescope "power" or magnification. A riflescope of the power 8 magnifies an image to eight times the size it would be when viewed by the normal, unaided human eye. The level of power affects the brightness of an image, so the lower the power of a riflescope, the brighter the image it delivers will be. In general, increasing power will reduce both field of view and eye relief, which are also discussed here.

 

 

 

 

 

 

 

 

 

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Diameter of Objective Lenses

The objective lenses of riflescopes are the front lenses. The diameter of one of these lenses, given in millimeters, will be the second number describing a particular riflescope. An 8x42 riflescope has an objective lens of 42mm. The diameter of the lens determines the light gathering ability of the instrument, with the greater light gathering ability of a larger lens translating into greater detail and image clarity. This is especially useful in low light conditions and at night.

Doubling the size of the objective lenses quadruples the light gathering ability of the riflescope. For instance, a 7x50 riflescope has almost twice the light gathering ability of a 7x35 riflescope and four times the light gathering ability of a 7x25 riflescope. This might lead you to assume that bigger is better when it comes to the diameter size of the objective lenses, but in reality the size of the lens must be considered along with exit pupil and intended usage to determine the best riflescope for you.

 

 

 

 

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Field of View

The size of the area that can be seen while looking through a pair of riflescopes is referred to as the field of view. The angular field of view is indicated on the outside of the riflescope, in degrees. The linear field of view refers to the area that can be viewed at 1,000 yards, and is expressed in feet. A larger field of view translates to a larger area seen through the riflescope.

Field of view is related to magnification, with greater magnification creating a smaller field of view, in general. A large field of view is especially desirable in situations where the object viewed is likely to move, a large terrain area is viewed, or when the user is moving.

You can use angular field to calculate the linear field by multiplying the angular field by 52.5. For example, if the angular field of a particular riflescope is 8° then the linear field will be 420 feet, i.e. the product of 8 x 52.5. Or, you can divide the linear field by 52.5, e.g. the riflescope has a field of view of 390 feet at 1,000 yards, so 390 divided by 52.5 is 7.43°

 

 

 

 

 

 

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Exit Pupil

The diameter, in millimeters, of the beam of light that leaves the eyepiece of a riflescope is the "exit pupil". The larger the exit pupil, the brighter the image obtained will be. Having a large exit pupil is advantageous under low light conditions and at night. For astronomical, marine or low light applications, the exit pupil of the riflescope should correspond with the amount of dilation of your eye's pupil after it has adapted to the dark. This number will be between 5mm and approximately 7.5 mm. 7 to 8mm of dilation is normally the maximum amount for the human eye, and this number tends to decrease with age.

To calculate the exit pupil, divide the size of the objective lens by the magnification of the riflescope. For example, the exit pupil of 7x50 riflescopes is 50 ÷ 7 = 7.14mm.

 

 

 

 

 

 

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Eye Relief

This refers to the distance, in millimeters, that a riflescope can be held from the eye and the full field of view can still be comfortably observed. Eyeglass wearers in particular benefit from longer eye relief. Normally, a minimum of 15mm will be needed to use the riflescope while wearing eyeglasses.

 

 

 

 

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Parallax

Parallax presents itself as the seeming movement of the reticle, in relation to the target, when your eye moves off center of the sight picture. It indicates that the scope is out of focus. Even when parallax-adjusted for a designated distance, there is an inadvertent error at other distances. Parallax worsens at higher magnifications.

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Center Tube Diameter

The diameter of a scope's center tube (or main tube) impacts the overall strength and durability of the scope. And it obviously determines the size of bases and rings required for mounting. But beyond that, the center tube diameter must be adequate to allow a sufficient range of windage and elevation adjustment.

 

 

 

 

 

 

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Length

The overall length of a scope, from the leading edge of the objective lens housing to the back edge of the eyepiece housing, can be a determining factor when selecting a scope model. Brush and horseback hunters may favor shorter scopes for their discreet profile. But long-range shooters, such as antelope hunters, might embrace longer models that also deliver greater magnification.

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Weight

The type of firearm you hunt with should be a determining factor where the weight of the scope is concerned. Lightweight mountain rifles, for example, seem to beg for scopes that weigh in at the low end of the spectrum. While more traditional rifles do well with the somewhat heavier options.

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Straylight

When light entering the scope reflects off of air-to-glass surfaces, the reflected light eventually exits in the scope in the form of stray light. This unfocused light typically diminishes the image quality of the sight picture.


 

 

 

 

 

 

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Lens Coatings

The optical elements of the riflescope are coated to reduce internal light loss and glare, which in turn ensures even light transmission, resulting in greater image sharpness, brightness, color fidelity and contrast. Choosing a riflescope with good lens coatings will ensure greater satisfaction with the riflescope you select. Lens coatings range in quality or applications as follows: coated -- fully coated -- multicoated -- fully multicoated. Coated lenses are the lowest performance and basically will not result in much satisfaction. Fully coated lenses are quite economical and can work well for you, depending on your needs. Multicoated or fully multicoated lenses are both very good choices. Fully multicoated lenses give the best light transmission and brightest images, and are therefore the most desirable.

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Construction

A critical factor in the performance of any riflescope is its construction. The security of the barrel alignment and proper internal mounting and alignment of the optics are crucial to producing a riflescope that's mechanically reliable, smooth functioning and long-lasting. The proper design, materials and construction will also assure that the riflescope is sealed to not allow intrusion of water or moisture.

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