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Glossary We hope that the following glossary of main terms and quality criteria for the
evaluation of binoculars and spotting scopes will be helpful to you in making a purchasing
decision. In addition, here are authorized Leica dealers and representatives around the
world who will be pleased to assist you. They have a large selection of products and they
will gladly present you your favorite model.
Technical binocular terms
Magnification Every binocular is specified by two numbers, of which the first one always indicates the magnification.
Magnification tells us how much closer an object appears to the observer when viewed through the binocular. For example :
At 8x magnification, a bird that is 328 feet/100 m away appears to be only 41 feet/12.5m (328 ft : 8 =41ft) away.
Front lens diameter The second characteristic number of a binocular is the front lens diameter
(the entry pupil of the binocular) stated in millimeters. A binocular with the designation10x 50 has
a front lens diameter of 50mm. The larger the front lens diameter, the more light can be gathered
by the binocular. If observation is to be conducted at twilight or at night, the ideal choice is a front
lens diameter of 42 or 50mm. Binoculars with a front lens diameter of 20 to 32mm are suited
primarily for observation in daylight.
Exit pupil The two bright circular areas that can be seen when one looks at the
eyepiece of a binocular from a distance of approximately 12inches/30 cm. (Exit pupil
= front lens diameter in mm : magnification factor). An indication of quality is that
these areas should be exactly circular and that they should have sharp edges. The
pupil of the eye changes its diameter in accordance with the light intensity (small in
bright light, large in the dark). For example, a 40-year old observer typically has a
maximal pupil diameter of 6mm. A rule of thumb for comfortable observation is that
the exit pupil of a binocular should be at least as large as the pupil of the eye at its
largest diameter. The exit- and entry pupils are not, however, the only decisive quality
characteristics for the evaluation of image brightness : Factors such a contrast,
resolving power and light transmission of a binocular or spotting scope are just as
important.
Twilight factor The twilight factor is a computed number
that describes the theoretical performance capability of
a binocular at twilight. It is computed by taking the square
root of the product of the magnification factor and the
front lens diameter. The twilight factor is a purely mathe-
matical value that says nothing about the optical quality
of a binocular, such as contrast, neutral color rendition,
resolving power, etc.
front lens diameter
magnification
Example LEICA TRINOVID 10x 42 BN
Magnification = 10 x, front lens diameter = 42 mm
Exit pupil = = 4.2 mm
Twilight factor = front lens diameter x magnification factor= 20.5
Objective field of view This value defines the width of
the field of view at a distance of 1000 meters. Usually,
the greater the magnification, the smaller the field of view.
Binoculars with a large field of view enable the observer
to cover a large area and to follow moving subjects, such
as a flock of birds, conveniently. A significant quality
criterion of Leica binoculars is an image that is a field of
view that is crisp and sharp to its very edges and that is
rich in contrast.
Great Grey Qwl/Strix nebulosa