Binocular Terminology
What do the numbers mean?
Binoculars are normally identified by two numbers. The first
number is the magnification and the second number is the diameter
of the objective lens in millimeters (mm). For example, 7x35 means
the binocular magnifies objects 7 times and the objective lens is
35 mm in diameter. A 10x50 binocular magnifies objects 10 times
and has an objective lens diameter of 50mm.
Prisms
There are generally two categories of prisms, porro prisms and
roof prisms. Porro and roof prism binoculars are distinguished by
their shape. Porro prism binoculars tend to be larger and have a
definite offset to their body design. Roof prism binoculars are
more straight and streamlined. Each provide excellent viewing.
Field of View
Describes the size of the area that can be seen through the
binocular or spotting scope. Expressed as the number of feet per
1000 yards of distance, the higher the magnification the narrower
the field of view. On wide angle binoculars (designated WA on
Alpen binoculars) a special lens design allows a wider field of
view than normal. An example field of view for a standard 7X
binocular would be 372 feet, whereas a wide angle would be 487
feet. Wider field of views are often preferred by hunters who want
to scan large areas for game or for viewing sporting events.
Objective Lens Diameter
This is important because many of the performance characteristics
of a binocular are determined by the diameter of this lens. The
function of the objective lens is to collect light and direct it
to the prisms or eye pieces. The larger the diameter, the more
light will reach your eye thus providing a brighter and sharper
image. However, the larger the diameter, the heavier and bulkier
the binocular will be. You will find there are many different
combinations of magnification and objective lens diameters to meet
specific applications.
Eye Relief
Eye Relief is the distance, expressed in millimeters, from the
ocular (eyepiece) lens to the point where the eye is positioned to
view the entire image. Eye relief is affected by magnification,
the number of lens elements, and field of view. Binoculars
designed with long eye relief are usually preferred by those who
wear eyeglasses. Most binoculars provide 8-13mm of eye relief.
Binoculars with "long eye relief" provide from 14-20mm (or more)
eye relief.
Exit Pupil
A measurement of how much light is usable by a binocular or
spotting scope. Generally, the larger the exit pupil, the brighter
the image will appear, particularly in dim light. Exit pupil is
determined by dividing the diameter of the objective lens by the
magnification. For example, a 7x35 binocular has an exit pupil of
5 mm. A 10x50 also has an exit pupil of 5 mm, so these are about
equal in dim light. A 7x50 binocular however has an exit pupil of
7 mm. For general use in bright light, an exit pupil of 2.5 mm is
sufficient and 4 mm and higher generally allow better viewing in
dim light. Lens coatings and the human eye also affect how well a
binocular performs in dim light.
Resolution
Describes how sharp or clear an object appears when viewed.
Resolution is primarily determined by the objective lens diameter.
The larger the diameter of the objective lens, the better the
resolution. However, lens coatings, lens alignment, quality of
glass, and quality of prisms also affect how sharp a viewed image
will be.
Coated Optics
Quality optics are coated to prevent glass surfaces from
reflecting light and loosing it. The very best optics are "Fully
Multi-Coated" which means multiple coatings are layered fully onto
all glass surfaces. "Fully coated" means all "air to glass"
surfaces have been coated with at least one layer of coating.
"Coated" means some surfaces have been coated.
