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How to Select
the Best Viewing Partner!
The following are several important yardsticks to
help you select the pair of binoculars that best suites
your requirements.
Roof Prism vs. Porro Prism
Both roof- and porro-prism optics are designed to
turn the image upside down — from an inverted
image to an erect image — by incorporating convex
optical elements in their objective and eyepiece
lenses. However, each design has some definite
advantages.
Featuring a straight-line axis and high-precision
optics, the roof-prism design is better equipped for
specialized applications. At the same magnification,
it also allows for a more compact, streamlined body
for enhanced maneuverability and portability.
On the other hand, the porro-prism design combines
time-honored optics and solid viewing performance
with a familiar, functional styling. It is customarily
employed in popular, multi-purpose models.
Magnification
The term “magnification” refers to the degree at
which your subject appears larger through binoculars
than with the naked eye. At eight-times
magnifi-cation, for instance, a subject standing at
800 meters (approx. 890 yards) away appears as if it
were standing at a distance of 100 meters (approx.
89 yards). The greater the magnification, the less
bright the image and the narrower the angle of
view. Since high-magnification models also increase
the risk of binocular shake, use of a tripod is
recommended for these models. If your application
involves occasional high-magnification viewing, a
zoom model may be
a good option.
Image Brightness
When you hold your binoculars at 30 centimeters
(approx. one foot) away from your eye, you will see
a bright spot, or “exit pupil,” in the middle of the
eyepiece lens. The larger the diameter of the exit
pupil, the brighter the image field. As a simple
yardstick, a diameter of three millimeters assures
comfortable daytime viewing, while that of five to
seven millimeters is required for astronomical
observation.
The exit pupil can be easily calculated by dividing the
effective aperture of the objective lens by the
magnification, while the relative brightness can be
obtained by squaring the exit pupil. For the same
magnification, binoculars with a larger exit pupil
offer a brighter image field and are better equipped
for observations under poor lighting conditions at
dawn or dusk.
Angle of View
Expressed in degrees, the term “angle of view” is
a yardstick for the image field that can be seen
without moving the binoculars. The greater the
magnification, the narrower the angle of view. So
binoculars with a greater angle of view are
recommended for observation of fast-action sports
and active subjects, while a pair with a greater
magnification is better suited for observation of
subjects at great distance.
If the subject is at a distance of 1,000 meters (approx.
1,110 yards), for instance, binoculars with a
five-degree angle of view offer a circular image field
of 87 meters (approx. 97 yards) in diameter.
Objective lens
Objective lens
Roof prism
Eyepiece lens
Eyepiece lens
Porro prism
Roof Prism
Porro Prism
A 10x50 model with a 5 real angle of view
100m/110 yd.
(= 1,000m 10)
Apparent angle
of view 50
1,000m/1,110 yd.
Angle of view 5
87m/97 yd.
