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)
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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