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Terminology - Are you needing to buy some new binoculars, but are confused about
all of the big words and foreign terminology that goes with all of the specs and numbers?
Read through this terminology guide so you can be confident you are getting the
correct binoculars for you particular usage. |
What 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 millimeter (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. A third type
of binocular known as galilean, does not use prisms but instead uses a set of lenses to
control magnification and focus.
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 pet 1000 yards of
distance, the higher the magnification the narrower the field of view. On wide angle
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 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 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 10 x 50 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.
Collimation is a word which describes the alignment of the two sides of
the binocular. Both sides must be aligned to provide a clear and stress free view.
Collimation is set at the factory using specialized optical alignment equipment. A product
can be out of collimation from being dropped or banged on a hard surface.
Fixed Focus or focus free binoculars were developed to allow the user to
quickly view images without having to adjust the focus wheel. These binoculars are pre-set
at the factory to allow for clear viewing from a distance of about 0-75 feet and beyond.
There is no focus wheel or eyepiece adjustment on such binoculars.
Interpupilary Distance (IPD adjustment) refers to the distance between
the right and left barrels of the binocular. This distance can be adjusted by simply
moving the barrels closer together or further apart until you see one circle of view while
looking through the binocular.
Compact Binoculars are designed with smaller prisms and objective lenses
to provide a more light weight and easily transportable binocular. These products
generally have objective lenses from 21 mm to 25mm in diameter.
Zoom Binoculars are designed with a special movable lens mechanism which
allows the user to go from a lower to higher magnification by simply moving a lever. |
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