MAGNIFICATION NUMBERS
Measured by two numbers separated by an “x” (8x40). The first number is the
power or the
magnification. With an
8x40, the object being viewed appears to be eight times closer than you would see it with the unaided eye.
The
objective lens size or the second number (8x
40) is the front lens diameter, which affects the brightness and sharpness of an image. The higher the number, the brighter and sharper it is.
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FIELD OF VIEW
The
field of view (FOV) is the side to side measurement of the circular viewing field or subject area. Magnification and eyepiece design determine the field of view. Generally, the higher the magnification, the narrower the field of view.
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EXIT PUPIL
The
exit pupil is the actual width of the beam of light that leaves the eyepiece. Divide the objective lens diameter (first number) by the power (second number) to calculate the size of the exit pupil. An 8x32 model has an exit pupil of 4mm. Large exit pupils supply brighter images under very low light conditions.
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TWILIGHT FACTOR
A number used to compare the effectiveness of binoculars or scopes used in low light. The
twilight factor is found by multiplying the size of the objective lens (in mm) by the magnification and then finding the square root of that result. Theoretically, the larger the twilight factor, the more detail you can see in low light.
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RELATIVE BRIGHTNESS (Relative Light Efficiency)
Relative brightness measures image brightness and is simply the exit pupil squared. A binocular with an exit pupil of 4mm will have a relative brightness of 16.
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PRISMS
A
prism is used to produce an image that is upright and correct left and right, otherwise one would have an upside down image.
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Porro Prism
A porro prism is an older style of prism used in optical instruments.
Porro prisms provide a greater depth perception and generally offer a wider field of view. A porro prism is larger and bulkier than a roof prism and the eyepiece and the front lens are never in a straight line but a porro prism is less expensive to make than a good roof prism and is every bit as good optically as a quality roof prism and at a much lower cost.
Roof Prism
A roof prism is a smaller prism shaped like a little house with a roof. The main advantage of a
roof prism is that it is smaller and more compact than a porro prism so it can be installed in a much sleeker body than a porro prism. Keep in mind though that a roof prism is generally much more expensive to make to the same optical standards as a good porro prism.
Bak4/BK7 Prism Glass
This refers to the grade of optical glass used in the prisms. All quality prisms use
Bak4 (
barium crown) glass that yield brighter images and higher edge-to-edge sharpness. The inferior
BK7 (
borosilicate) grade of glass is used in inexpensive binoculars.
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LENS COATINGS
Lens surface coatings are chemical coatings on the lenses that reduce light loss and glare and make images brighter with less eyestrain. Whenever light strikes a lens, a certain percentage of light is lost so a lens with no coating would lose incoming light.
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Coated
A “
coated” lens has a single layer on at least one lens surface.
Fully Coated
A "
fully coated" lens is coated in a single layer of magnesium fluoride on all air-to-glass surfaces -the oldest type of lens coating. This coating is found only on inexpensive sports optics and will not produce images as bright as multi-coated binoculars.
Multi-Coated
A “
multi-coated” lens is the best lens because it uses multiple layers of special chemicals on at least one lens surface. This type of coating allows the maximum amount of light to pass through the lens and is brighter than a “fully coated" lens.
Fully Multi-Coated
A “
fully multi-coated” lens has multiple layers on all air-to-glass surfaces. This type produces the brightest image of any coating system. All quality products will be "fully multi-coated".
FOCUSING
Center Focus
A
center focus uses a single wheel to focus on objects. It is the most versatile and commonly used focusing system in a binocular.
Individual Eyepiece Focus
An
individual eyepiece focus requires you to focus each eyepiece when looking at an object, but once focused for your eyes, no additional focusing is required on objects from 40 yards away and greater.
Focus Free
A "
focus free" binocular is one where the eyepieces are locked and set at the factory and cannot be adjusted. This means that you can never focus on objects closer than forty yards away and it also means it cannot be adjusted for differences in strength between your right eye and left eye.
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INTERPUPILLARY DISTANCE
The
interpupillary distance is the distance between the pupils of the eyes.
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DIOPTER ADJUSTMENT
The
diopter adjustment is a “fine focus” ring that allows you to compensate for differences in strength between your right and left eye.
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CLOSE FOCUS / MINIMUM FOCUS
The "
minimum focus" or "
close focus" is the nearest distance at which a sports optic will focus on an object.
EYE RELIEF
Eye relief is the maximum distance your eye can be from the eyepiece and still see the entire field of view. Eye relief is of primary concern for people who wear eyeglasses. An extended or long eye relief reduces eyestrain.
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EYECUPS
Eyecups should be put in the "down" position when wearing eyeglasses and in the "up" position when not wearing eyeglasses. Most modern sports optics have "
twist up" or "
pull up" eyecups.
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WATERPROOF
A
waterproof optic will not fog up due to moisture inside. A sports optic is made
waterproof and
fogproof by nitrogen purging and by being O-ring sealed. These models can withstand complete immersion in water and stay dry inside.
ARMORING
Housing covered by rubber or other synthetic material is what we mean by
armoring. Armoring protects against scratches and bumps from day to day use. It provides a comfortable gripping surface which makes it easier to hold, and amoring "quiets" the sport optic when it accidentally bangs against something so it doesn’t spook wildlife.
TRIPOD MOUNTING
Any time you have a sports optic magnification over 10x or 12x, you should attach it to a
tripod to steady the image. Also, heavy optics with a last number of 70 mm or more usually need a tripod to support the weight.
USING THE DIOPTER / FOCUSING YOUR BINOCULARS
Most binoculars feature a center focus wheel and also an adjustment ring (either on the right eyepiece or integrated into the center focus wheel) known as the diopter. The diopter adjusts for differences between your individual eyes (many people have one eye that is “stronger” than the other). The procedure for attaining proper focal balance using the center focus wheel and the diopter is:
1. With your right eye closed (or with your hand over the right objective lens), focus your left eye on an object approximately 20 yards away with the center focus wheel until it is in sharp focus.
2. Now adjust for your right eye. To do this, close your left eye (or, again, place your hand over the left objective lens) and focus your right eye on that same object 20 yards away. Focus using the diopter until the object is sharply focused. Note: Some diopters have a locking feature that you’ll need to unlock before moving the diopter ring.
3. The binoculars are now set for your eyes. Make a note of your diopter setting if you are sharing your binoculars with someone else.