 Binoculars FAQs
Considerations in choosing a pair of binoculars.
The brilliance and sharpness of the image you see through a particular
binocular or spotting scope is determined by a number of different
factors, including the interaction of these factors. Magnification,
optical coatings and lens diameter are just a few of the factors
influencing how a binocular performs. Of course, the vital role
in performance is played by the quality of the optics. All of
our optics deliver excellence through careful consideration of
quality in the glass and lens coatings used, precision manufacturing
processes, and uncompromising quality control.
What do the letters and numbers in the binoculars'
name refer to?
"B" - means binocular;
"P" - prismatic with 'Porro' reverse magnification system;
"C" - binocular with central focusing system;
"O" - with magnified exit pupil distance;
"K" - with reverse roof prism system;
"F" - with inside focusing of the eyepiece;
_S_ _with wide angle lenses for a wider field of view.
The first number represents the magnification of the binoculars
and the last number states the objectives diameters in millimeters.
What are common differences in binoculars?
Just like your eyes, binoculars use prisms to create a properly
oriented image. The quality of the layout and materials affects
the images that the binoculars can provide. Barium crown glass
is a high density glass that eliminates internal light scattering
and produces sharp images with clear, circular exit pupils. The
glass quality also affects how clearly an image appears across
your viewing area. Properly ground and installed glass creates
a flat image with crispness and clarity across the full width
of the image.
Optics Coatings.
When optics are coated with an anti-reflection coating, the amount
of light reflected by the glass is reduced. This allows more light
to actually reach your eye thus giving you a brighter, clearer
picture. The better the coating, the higher the binoculars' light
transmittance. Generally coating schemes are notated as such:
Coated optics: One or more surfaces of one or more lenses have
received an anti-reflective coating. Fully coated optics: One
or more surfaces of one or more lenses have been coated with multiple
films. Some surfaces could be single coated or some not coated
at all. Fully multi-coated optics: All air-to-glass surfaces should
have received multiple films. Choosing a binocular with good lens
coatings will translate to greater satisfaction with the product
you ultimately select. When it comes to binocular coatings you
get what you pay for so it is worth a little extra money now for
greater enjoyment throughout the life of the binocular.
Field of View.
The width of the image at its widest point is the field of view
of the binoculars. Fields of view can both be measured at the
end of the binoculars or at a measured distance giving a field
of view at 100 or 200 meters. The result will be given in either
feet or degrees. The field of view is limited by the eye piece
and not simply the size of the exit lenses. The field of view
decreases as magnification increases. Many special models with
wide angle lenses which give a wider field of view are produced
to overcome narrow fields of view in mid- to high- magnification
binoculars. Such wide-angle binoculars are better for giving you
the peripheral vision necessary for watching sporting events or
picking up game and targets in hunting and paint balling. The
larger prisms and eyepieces of these glasses add weight and bulk
to a binocular and there may be a loss of sharpness at the edge
of the field. Eyeglass wearers usually have difficulty seeing
the full field of view through wide-angle optics and should consider
binoculars featuring longer eye relief. The angular field of view
is indicated on the outside of the binocular, in degrees. The
linear field of view refers to the area that can be observed at
1,000 yards, and is expressed in feet. You can use angular field
to calculate the linear field by multiplying the angular field
by 52.5. For example, if the angular field of a particular binocular
is 8ø then the linear field will be 420 feet, i.e. the product
of 8 x 52.5.
Aperture.
The size of the objective lens or big end is related to the size
of the binocular. A larger objective lens will gather more light
allowing you for greater image detail. A small difference in aperture
has a big effect on the light gathering ability of a binocular.
The apertures of compact models generally range from 20mm to 25mm
while _giant binoculars_, used mainly for astronomy, may range
from 70mm to 80mm in diameter.
Magnification.
Magnification is the extent that the image is being enlarged.
A great example is Kalinka Optics wonderful BPOC 10x42 Military
Commander's Binoculars. The number 10 represents the magnification
or power of the binocular. A binocular of this power enlarges
an image ten times. The power of a binocular does have an effect
on the brightness of an image, so the lower the power of a binocular,
the brighter the image it delivers will be. For the same size
and style of binocular, increasing the power or magnification
of the binocular will let you see further but reduce the field
of view and eye relief. Hand-held binoculars range from 2x to
30x. As magnification increases, size and weight generally follow.
Many larger binoculars with even higher magnification may need
to be mounted on a tripod. SO while people often think of bigger
being better, remember that brightness and clarity may diminish,
the depth of field may be shallower and the field of view is usually
more restricted. Fine hand tremors, distortion by heat waves and
other effects of atmospheric conditions are also more noticeable
at higher powers.
Examples of Different
Magnifications
  
  
Porro Prism Binoculars.
Traditional Porro prism models have off-set barrels that are larger
and bulkier than roof prism models. Modern designs and production
methods now make it possible to produce smaller Porro prism binoculars
that may actually weigh less than a roof prism of the same configuration.
The reverse Porro prism design reduces the overall size of a binocular
and is commonly used for compact models. This binocular design
can deliver a wide field of view with excellent image sharpness.
Roof Prism Binoculars.
Roof prism models are generally smaller and more streamlined than
Porro prism binoculars. They have straight barrels and a compact
optical design that is able to tolerate rough treatment better.
Higher quality models have an internal type of focusing mechanism
which gives them a high degree of structural integrity. They are
also less susceptible to internal fogging and potential dust or
moisture entry. Top quality roof prism models require precise
tolerances when manufacturing their complex prism configurations
and often cost significantly more than Porro prism models of the
same quality.
Eye Relief.
Eye relief is the distance where images are projected from the
ocular lens to their focal point. Binoculars with longer eye relief
project the image through your glasses all the way back to your
eye. This distance can vary from as little as 5mm to as much as
23mm. Without this increased distance, the eyeglass wearer will
only see part of the field of view that is seen by those who don_t
wear eyeglasses. Models designed to give extended eye relief are
sometimes called high eye point binoculars. If you plan to use
your binoculars while wearing either eyeglasses or sun glasses,
you_ll find models with an eye relief of 15mm or more will provide
you with comfortable viewing. Keep in mind that many long eye
relief models will not have a particularly wide field of view.
Exit Pupil.
The circular beam of light that you can see in the eyepiece when
you hold the binoculars in front of you is the exit pupil. More
precisely, the exit pupil is the magnified image in the eyepiece
as it leaves the binocular to enter your eye. When the exit pupil
of the binocular is larger than the eye pupil, some of the light
coming from the binocular will fall on the iris and go undetected
by the observer. When the exit pupil is smaller than the eye pupil,
the amount of light falling on the retina will be less than that
collected with your normal vision. The image will appear dim and
since resolution and contrast are adversely affected, the image
appears less clear. Knowing the exit pupil can help you choose
a binocular that is well-suited to your activities. You may want
to use a binocular with a large exit pupil for activities like
boating, during which it is hard to keep a steady hand. (It is
much easier to keep the smaller pupil of the eye centered in the
larger exit pupil of a binocular). Viewing in bright conditions:
eye pupil ranges from 2mm to 3mm. Almost all binoculars gather
more light than is needed by your eye. You can look through a
compact 8x20 model (exit pupil of 2.5mm) and see an image nearly
as bright as that seen through a 7x50 model (exit pupil of 7.1mm).
Your eye pupil is the limiting factor in observed brightness.
Viewing at twilight: eye pupil ranges from 4mm to 5mm. For the
best viewing, you_ll want a binocular with a minimum exit pupil
of 4mm. 8x32 and 10x40 models both meet this standard. Viewing
in dark conditions: eye pupil ranges from 6mm to 8mm. You_ll want
a binocular offering the largest exit pupil you can handle. For
astronomy applications, the exit pupil of the binocular should
correspond with the amount of dilation of your eye's pupil after
it has adapted to the dark. This number will be between 5mm and
9mm. 9mm of dilation is the maximum amount for the human eye,
and this number tends to decrease with age. To calculate the exit
pupil, divide the size of the objective lens by the magnification
of the binocular. For example, the exit pupil of 7x42 binoculars
is 42/7 = 6mm.
Low Light Conditions.
During daylight hours the magnification of a binocular will be
the principal factor in image resolution. At night, when the eye
pupil is dilated, aperture size is the controlling factor. In
twilight conditions both of these factors affect resolution. Binoculars
have to be able to overcome the loss of light to provide clear
images in dim light conditions. A good way to get a feel for the
binoculars_ ability to do this is to multiply the magnification
by the aperture and then find the square root of this product.
The result should give you a basis for comparison between binoculars.
With this formula, you will find that a 10x40 model (twilight
factor of 20) would resolve better than a 7x35 model (twilight
factor of 15.4) even though the 10x40 has a smaller exit pupil.
Remember, however, that the twilight factor will primarily indicate
performance at dawn or dusk without consideration of the light
transmittance or glass quality of the binocular.
Diameter.
The objective lenses of binoculars are the front lenses. The diameter
of one of these lenses, given in millimeters, will be the second
number describing a particular binocular. Hence, a 7x42 binocular
has an objective lens of 42mm. The diameter of the lens determines
the light gathering ability of the instrument, with the greater
light gathering ability of a larger lens translating into greater
detail and image clarity. This is especially useful in low light
conditions and at night.
Doubling the size of the objective lenses quadruples the light
gathering ability of the binocular. For instance, a 7x50 binocular
has almost twice the light gathering ability of a 7x35 binocular
and four times the light gathering ability of a 7x25 binocular.
This might lead you to assume that bigger is better when it comes
to the diameter size of the objective lenses, but in reality the
size of the lens must be considered along with exit pupil and
intended usage to determine the best binocular for you.
Resolution.
A measurement of the binocular's ability to distinguish fine detail
and produce a sharp image. Better resolution also delivers more
intense color. Resolution varies in relation to the size of the
binocular's objective lenses. Generally, a larger objective lens
will deliver more detail to the eye than a smaller objective lens,
regardless of the magnification of the binocular. Actual resolution
is determined by the quality of the optical components, the type
and quality of the optical coatings, atmospheric conditions, collimation
(i.e. proper optical alignment), and the visual acuity of the
user.
Contrast.
Refers to the degree to which both dim and bright objects in the
image can be differentiated from each other and from the background
of the image. High contrast helps in observing fainter objects
and in discerning subtle visual details. High quality optical
coatings provide better contrast in an image. The other factors
affecting contrast are: collimation, air turbulence, and objective
lens, prism and eyepiece quality.
Construction.
A critical factor in the performance of any binocular is its construction.
The security of the barrel alignment and proper internal mounting
and alignment of the optics are crucial to producing a binocular
that's mechanically reliable, smooth functioning and long-lasting.
Collimation.
The alignment of the optical elements of the binocular to the
mechanical axis. Good collimation prevents eyestrain, headaches,
inferior and double images while improving resolution. Unfortunately,
proper collimation is almost impossible to achieve in very low-priced
binoculars that lack quality components and design.
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