Binoculars & Binocular Terminology
The brilliance and sharpness of the image you see through a particular
binocular is determined by a number of different factors, including the
combination of these factors. Magnification, optical coatings and lens diameter
are just a few of the factors influencing how a binocular performs.
However, the single most important performance feature will always be the
quality of the optics. Steiner offers optical excellence through careful
consideration of quality in the glass and lens coatings used, precision
manufacturing processes, and uncompromising quality control.
Please consider the following factors when choosing a
Magnification is the degree to which the object being viewed is enlarged. For
example, with an 8x42 binocular, the number 8 represents the binocular "power"
or magnification. A binocular of the power 8 magnifies an image to eight times
the size it would be when viewed by the normal, unaided human eye. The level of
power affects the brightness of an image, so the lower the power of a binocular,
the brighter the image it delivers will be. In general, increasing power will
reduce both field of view and eye relief, which are also discussed here.
Diameter of Objective Lenses
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. An 8x42 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.
Field of View
The size of the area that can be seen while looking through a pair of
binoculars is referred to as the field of view. 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 viewed at 1,000 yards, and is expressed in feet.
A larger field of view translates to a larger area seen through the binocular.
Field of view is related to magnification, with greater magnification
creating a smaller field of view, in general. A large field of view is
especially desirable in situations where the object viewed is likely to move, a
large terrain area is viewed, or when the user is moving.
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 8 degrees then the linear field will be 420 feet, i.e. the product of 8 x
52.5. Or, you can divide the linear field by 52.5, e.g. the binocular has a
field of view of 390 feet at 1,000 yards, so 390 divided by 52.5 is 7.43 degrees.
The diameter, in millimeters, of the beam of light that leaves the eyepiece
of a pair of binoculars is the "exit pupil". The larger the exit pupil, the
brighter the image obtained will be. Having a large exit pupil is advantageous
under low light conditions and at night. For astronomical, marine or low light
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 approximately 7.5 mm. 7 to 8mm of dilation is normally
the maximum amount for the human eye, and this number tends to decrease with
To calculate the exit pupil, divide the size of the objective lens by the
magnification of the binocular. For example, the exit pupil of 7x50 binoculars
is 50 divided by 7 = 7.14mm.
This refers to the distance, in millimeters, that a binocular can be held
from the eye and the full field of view can still be comfortably observed.
Eyeglass wearers in particular benefit from longer eye relief. Normally, a
minimum of 15mm will be needed to use the binocular while wearing eyeglasses.
Inter Pupil Distance or "IP" Distance
This refers to the distance, in millimeters, that a binocular can be used by
users with both eyes viewing a complete viewing picture. In other words minimum
distance for viewers with narrow eye distance and the maximum distance for
viewers with eyes that are farther apart. Normal optics industry standards range
at a minimum of 58 to a maximum width of 72mm. To accommodate a wide range of
users, most Steiner models offer a minimum of 56 to a maximum of 75mm or more
The binoculars' ability to gather and transmit enough of the available light
to give a sufficiently bright and sharp image defines its brightness. The
brightness of a binocular also enhances color differentiation in the image
observed. R.B.I. (Relative Brightness Index), Twilight Factor and R.L.E.
(Relative Light Efficiency) are common indices used in the binocular industry,
but are all somewhat flawed in their design and often prove fairly meaningless
as both high quality and low quality binoculars will gather the same result.
Brightness is one criteria to be considered when purchasing binoculars, but is
not always the most important factor. Given in order of importance to the
overall brightness of a binocular, the following factors are worth
investigating: objective lens diameter, magnification, the type and quality of
the objective lens glass, type of lens coatings and type of prisms used. In
general, large objective lenses, low magnification and fully multicoated lenses
are the most desirable. At Steiner, all low light binoculars are tested to
perform to the highest "light transmission" this means that light is brought
into the objective side and then measured at the exit, the eyepieces. The
percentage remaining is "light transmission". Good low light binoculars will
have high light transmission across the whole spectrum of color, evenly to
produce good color fidelity and best low light results. Binoculars with spikes
in their transmission at certain wave lengths are designed for brightness and
contrast, Steiner also offers models for these types of specific applications.
Resolution is a measurement of the binoculars' 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 binoculars' 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.
However through technological advancements in production, there are a number of
compact models available that deliver detail as good as larger models. 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.
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. The same applies to differences in color especially in darker or
difficult light conditions. 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. Steiner offers
several high contrast coatings such as the Predator Series for spotting and
viewing wildlife against a foliated or wooded background or the Navigator Series
which reduce haze and excessive blue light to product a sharp high contrast
image while viewing over water or similar conditions.
The distance between the binocular and the nearest object you can focus on,
while maintaining a good image and sharp focus, defines the close focus distance
of a binocular. This term is normally applied only to center focus models which
may need to focus both near and farther distances. Close Focus distance is a
critical issue with bird watchers and observers of close subjects that tend to
move around at varying distances quickly. Steiner offers "Fast close focusing"
on its lines of roof prism binoculars including the Merlin, Wildlife Pro,
Peregrine and Predator Series.
The optical elements of the binocular are coated to reduce internal light
loss and glare, which in turn ensures even light transmission, resulting in
greater image sharpness, brightness, color fidelity and contrast. Choosing a
binocular with good lens coatings will ensure greater satisfaction with the
binocular you select. Lens coatings range in quality or applications as follows:
coated -- fully coated -- multicoated -- fully multicoated. Coated lenses are
the lowest performance and basically will not result in much satisfaction. Fully
coated lenses are quite economical and can work well for you, depending on your
needs. Multicoated or fully multicoated lenses are both very good choices. Fully
multicoated lenses give the best light transmission and brightest images, and
are therefore the most desirable. Steiner offers only multicoated and fully
multicoated lenses using a variety of proprietary mineral source substances to
produce outstanding viewing results.
A binoculars' prisms serve to invert the image and come in one of two basic
designs: Roof or Porro prisms. By design, roof prisms are more lightweight and
compact, for portability. Porro prisms are designated either BK-7 or BAK-4. Both
are economical and highly effective designs. The finer glass in the BAK-4 design
is of high density and virtually eliminates internal light scattering, producing
sharp, well defined images. Steiner uses only BAK-4 prisms in their binoculars.
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. This is a special strength in Steiner
models: By starting with the most durable design and then combining that with
very strong materials that will hold the optical elements in place securely not
allowing them to shift, therefore not to cause flaws or discomfort in the
viewing experience. The proper design, materials and construction will also
assure that the binocular is sealed to not allow intrusion of water or moisture.
Collimation is 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. All Steiner models are collimated to exceed universally
acceptable standards so as to produce a very comfortable viewing experience for
longer periods of time.