What is a binocular?
Binoculars magnify distant objects so that they appear nearer to the
viewer. A binocular essentially consist of a pair of parallel telescopes
joined at the center by a hinge. The hinge allows the eyepieces to be
brought to a suitable interpupil distance for each individual.
The above figure shows a classical design. The objective lens
gathers light from the object you are viewing. It consists of two glass
elements glued together. In optics jargon, this is called a compound
lens which reduces distortion. The light then passes through the Porro
prism (or roof prism in a more compact design). The prism
serves two purposes: it produces erect images and folds the light path
to reduce the physical size of the binoculars. Light then passes through
eyepieces and enters your eye. One of the two eyepieces is usually
adjustable to adaptor to each user. Both eyepieces are adjusted together
by center focusing.
All lenses are usually optically coated to reduce scattered light.
One of the most popular coatings on the objective lens is ruby coating,
which provides both performance and durability.
The numbers:
Binoculars are specified mainly by two numbers, e.g., 8 x40,
the first for the factor of magnification and the second the
objective lens diameter in millimeters.
For any practical application, magnification usually ranges
from 3 to 10. Anything higher than a power of 10x is not desirable for
several reasons: (i) the amount of light reaches the eyes decreases
with increasing power so that the object appears darker at higher magnification;
(ii) the field of view decreases with increasing power; and, most importantly,
(iii) the higher the power, the more sensitive viewing is to any little
movement. As a matter of fact, viewing through a pair of 15x binoculars
is an exhausting experience because even your heart beat or breathing
with make the image dance around!
A larger objective lens diameter means more light enters your
eyes. However, a larger objective lens also means a larger and longer
binocular body. If you want to carry around a pair of binoculars, a
suitable objective lens diameter is between 20 and 50 mm.
Another parameter is the field-of-view, which is usually given
as degrees or as length at 1000 yards (shown below). The field of view
usually ranges from about 2 to 10 deg, depending on design.
Here are some further readings you can do
if you really want to know everything:
[1] American Institute of Physics, "The
Binoculars" (1975).
[2] B. Schwalberg, "A Buyer's Guide to Binoculars", Popular
Photography, (June, 1987).
[3] J. W. Seyfried, "Choosing, using, & repairing binoculars",
(University Optics Inc., Ann Arbor, 1995). |
