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| What do the Binoculars Numbers
refer to? |
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| The first number refers to the magnification of the
binoculars, the second number indicates the size of the objective lens, the
aperture of the binoculars. |
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| Magnification |
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| The first number in a binoculars' designation indicates how
much larger, or closer, an image will appear. This magnification is
often indicated with an "x", indicating the "power" of the binocular.
For example, an image will appear 8 times larger through 8 x 40 binoculars
than when viewed with the naked eye. More magnification is not necessarily
better. As magnification increases, brightness and clarity may
diminish, and the field of view is usually more restricted. |
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| Objective Lens/Aperture: |
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| The second number of a binoculars' designation refers to the
diameter of the objective lens (the larger end of the binoculars). It is
almost always directly related to the size and weight of the binoculars.
Compact binoculars will have objective lenses ranging from 15mm to 25mm in
diameter, while truly "full-size" binoculars' objective lenses range around
35mm to 42mm. A larger objective lens will gather more light and,
theoretically, provide you with greater detail and a clearer image.
The trade-off with larger objective lenses is that the binoculars themselves
become rather large and can be unwieldy for some users. |
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| Different designs of binoculars: |
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| Porro prism binoculars: |
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| The classic look of binoculars, shaped like the letter M.
The Porro prism design can deliver a wide field of view with excellent image
sharpness at very affordable prices. Traditional Porro prism
binoculars have off-set barrels that are large in size and can be bulky.
In compact models, the prisms are designed differently in order to reduce
the overall size of the binoculars. |
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| Roof prism binoculars: |
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| A newer prism design, roof prism binoculars are generally
smaller and more streamlined than Porro prism binoculars. They have
straight barrels and a compact optical design that is able to tolerate rough
treatment better, too. Roof prism models require precise tolerances
when manufacturing, and their complex prism configurations often mean that
they cost significantly more than Porro prism models of the same quality |
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| Exit Pupil |
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| The exit pupil of a binocular is the point at which all of
the light rays that entered the objective lens and passed through the
binocular exit through eyepiece to form a magnified, circular image.
If you hold the binocular away from your eyes and look through the ocular
lens, you will be able to see the clear circular exit pupil. |
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| Eye Relief |
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| Eye relief in a binocular refers to the distance images are
projected from the ocular lens to their focal point. The eye relief of
a binocular can vary from 5mm to as much as 23mm. Eye relief is of
importance to eyeglass wearers, who will need a minimum of 14-15mm of eye
relief to be able to use binoculars effectively. A binoculars' eyepieces
will have either rubber, fold-down eyecups or more modern retractable style
eyecups for accommodating the user, whether they wear eyeglasses or not. |
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| Field of View |
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| The field of view is the size of the area you can see with a
pair of binoculars. This measurement is expressed as the width,
measured in feet, of the area you would see at 1000 yards. |
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| Close Focus |
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| The close focus of a binoculars refers to the minimum
distance which it will focus to. The close focus of a particular model
depends on its optical design and, to a small extent, on your own ey'es
characteristics. For many birdwatchers and butterfly watchers, it is
desirable for binoculars to focus to at least 10 feet. |
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| Interpupillary distance (IPD) |
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| The distance from one eye pupil to the other, expressed in
millimeters. For many people, IPD is not a consideration, but for
people with either narrow-set or wide-set eyes it may be important to know
your personal interpupillary distance. |
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| Twilight factor |
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| A commonly used measurement of viewing efficiency and image
detail in low lighting conditions. |
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