in Choosing 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
However, the single most important criterion in performance
will always be the quality of the optics. Celestron delivers 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 a 7x42 binocular,
the number 7 represents the "binocular power". A binocular of the power 7
magnifies an image to seven 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.
Magnification Comparison Chart
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
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 observed at
1,000 yards, and is expressed in feet. A larger field of view translates to a
larger area seen through the binocular.
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, or when the user is
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° then the linear field will be 420 feet, i.e. the
product of 8 x 52.5.
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 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.
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 =
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.
binocular’s 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. Brightness
is one criteria to be considered when purchasing binoculars, but is not 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 most
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.
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.
The distance between the binocular and the nearest object you can
focus on, while maintaining a good image and sharp focus, defines the near focus
of a binocular.
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 and contrast. Choosing a binocular with good lens coatings will
translate to greater satisfaction with the product you ultimately select. Lens
coatings range in quality as follows: coated -- fully coated -- multicoated --
fully multicoated. Coated lenses are the lowest quality and basically will not
result in a product that will satisfy you. 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
binocular's 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.
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.
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.
As you see, there are a number of different factors to
consider in choosing a binocular. Your Celestron dealer will be able to assist
you in making the choice that's right for you and that will bring you years of
viewing comfort and pleasure.