90 / 91
60 x
30 x
10 x
7 x
W
a
terbuck / K
obus ellipsipr
ymnus
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, there are authorized Leica dealers and representatives around the
world who will be pleased to assist you. They have a large selection of products.
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 8 x magnification, a bird that is 328 feet/100 m away appears to be only 41 feet/12.5 m (328 ft : 8 = 41 ft) 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 designation 10 x 50
has a front lens diameter of 50 mm. The larger the front lens diameter, the more light can be gath-
ered 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 50 mm. Binoculars with a front lens diameter of 20 to 32 mm 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 12 inches/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
maximum pupil diameter of 6 mm. A rule of thumb for low light 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 charac-
teristics 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 squa-
re 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.
Objective field of view
This value defines the width of
the field of view at a distance of 1,000 meters or 1,000
yards. 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.
front lens diameter
magnification
Example LEICA ULTRAVID 10 x 42 BR
Magnification = 10 x, front lens diameter = 42 mm
Exit pupil =
= 4.2 mm
Twilight factor = front lens diameter x magnification factor = 20.5
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