|A good pair of binoculars is essential equipment on
any boat or outdoor adventure. However, buying binoculars can be overwhelming with all the technical information
available. The following guide defines the key terms and
specification details used.
Two numbers identify binoculars, the first number given
represents the "magnification power" and the second number is the
"objective lens diameter".Magnification Power:
The first number indicates how much closer (or how many times
larger) an item will appear when compared to the unassisted eye. For
example, if you view a buoy that stands 350 yards away through 12X
binoculars, it will appear as though it were 29.2 yards away (350
divided by 12).
It is important to remember though, that while
a higher magnification power seems the way to go, as this number
increases the movement of your hands will be amplified making steady
viewing more difficult.
Diameter: The objective lens is the large lens at the front
end of the binoculars; this second number refers to the size of the
objective lens in millimeters. Since this lens determines how much
light is captured, the larger the objective lens, the brighter the
view will be, especially in low light conditions.
In addition to magnification power and objective lens diameter,
there are several other terms and measurements that should be
considered prior to your purchase.
This is the distance from your eye to the eyepiece at which a
complete image (the whole field of view) is visible with no image
cutoff. Longer eye relief is more comfortable as it allows you to
hold the binoculars away from your face.
A longer eye relief
spec is especially desirable if you wear glasses. Many manufacturers
recommend rolling down the rubber eyepiece collar before use with
Exit Pupil: The size of the image
at the eyepiece, or exit pupil. This number is calculated by
dividing the objective lens diameter by the magnification.
Generally, the larger the exit pupil, the brighter the image will
be. A large exit pupil will also make it easier to maintain a full
image if your hands are unsteady.
For better viewing in low
light, look for binoculars with an exit pupil of 4mm or
Field of View: An angle with the
viewer as the starting point, used to define how wide an area,
usually in feet, that can be viewed at a glance 1,000 yards away. A
higher power (magnification) will often have a narrower field of
view, a wider field of view makes object location
Brightness: Sometimes called
relative brightness, this number is equal to the square of the exit
pupil and indicates the quantity of light the binoculars transmit to
your eyes. A higher number will provide a brighter image, and a good
choice for nighttime viewing.
Applied to optical surfaces to reduce reflection and increase light
transmission. All binoculars have coatings at least on the objective
and ocular lenses (lenses closest to the eyes). The best binoculars
are those in which every optical surface is coated, preferably with
as many as 13 or more coatings each measuring less than a
Prism: Objective lenses form
images that are both upside down and reversed, prisms are utilized
in binoculars to correct the orientation. Two primary types of
prisms are used, porro or roof. When the roof prism is used in
binoculars, the result is a straight-line design while the porro
prism gives binoculars the zig-zag profile.
designed and manufactured, either type of prism provides excellent