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WHAT YOU NEED
TO KNOW BEFORE GETTING A PAIR OF BINOCULARS
What You Need To
Know Before Getting A Pair Of Binoculars
What Do The Numbers On
Binoculars Mean ?
Objective Lens Size
All binoculars use the
same method to define their technical specs. So for example a pair
of binoculars may be described as 10 x 50. So what does this mean?
The “10” refers to the
magnification power of the binoculars. This basically means that
objects viewed will appear to be 10 times (10x) closer than when
viewed with the bare eye.
The second number (in
this case “50”) indicates the diameter of the objective lens (the
light gathering lens at the front of the binoculars); this is
measured in millimeters. Note that the diameter of the lens is
directly related to the size of the binoculars—the larger the
objective lens, the larger (and normally brighter) the size of the
Field Of View
When you look through
your binoculars, the widest dimension you can see is known as the
field of view. So for example a field of view of 390´ (feet)
indicates that the width of the image you see is 390 feet at a
distance of 1000 yards. Some binoculars feature unique lenses to
provide a wide field that is greater than normally seen through
binoculars of the same magnification (awide-field pair of binoculars
is better for observing fast-moving objects, such as herds of racing
animals like antelope). You should also be aware that the field of
view diminishes as magnification increases.
You can see the exit
pupil when looking through the eyepiece of your binoculars held at
arm’s length—that little stab of light you see in the ocular lens,
is the exit pupil. So what’s its relevance?
The diameter of the
exit pupil determines how much light is transmitted to your eye…in
other words, it determines how much you get to see (this is mostly
of significance in low-light conditions).
The exit pupil is
determined by dividing the objective lens size by the magnification.
So for binoculars with specs. of 10 x 50 or 7 x 35 the exit pupil
size would be 5mm. this is how it’s determined:
lens size / magnification
Exit Pupil=50 / 10=5mm
Exit Pupil=35 / 7=5mm
The diameter of the
exit pupil does not matter much if there’s enough ambient light,
since the pupils of your eyes are generally smaller than the exit
pupil size of your binoculars. However with dimming of ambient
light, the pupils of your eye adapt by enlarging. This means that if
the exit pupil size of your binoculars is comparatively small, in
low light conditions this is going to be a significant restrictive
factor. Consider the following example:
A pair of compact
binoculars has the following specs: 8 x 25 i.e., magnification of 8
x and objective (front) lens of 25mm. As discussed above, we can
easily determine the exit pupil size;
Size=objective lens / magnification=25 / 8=3.1mm (approx.)
When ambient light
gets low enough the pupils of your eyes adapt by increasing in size.
If their size increase is greater than 3.1mm this means that size of
your pupils is now greater than the exit pupil size of your
binoculars…in simple terms this means that your binoculars aren’t
transmitting enough light to your eye. Now consider if your
binoculars had the following specs: 7 x 50
Exit Pupil Size=50 /
7.1mm diameter is the
same (slightly greater actually) aperture size of fully-dilated
pupils of eyes in excellent condition (typically found in young
people). With binoculars that possess similar specs. (7.1mm exit
pupil size), the amount of light transmitted to your eyes is never
an issue. You’ll always be able to see well in low light conditions.
Note that as we age
the ability of our eyes to adapt to dim light is diminished. Our
pupils no longer widen to the same extent as when we were younger.
In fact the pupils of the elderly generally don’t dilate beyond
4mm—so as one ages, the exit pupil size requirement decreases.
This refers to the
optical and mechanical alignment of binoculars. Cheap binoculars are
often factory-shipped out of collimation. Good binoculars are
carefully collimated, usually with laser precision. This translates
into time and added cost at the manufacturing level—which translates
into “more expensive” at your level (but it’s worth it in the long
binoculars will give you eyestrain and headaches.
Performance is a measure of the capability of binoculars in low
A quick way to
determine the Twilight Performance is to multiply magnification with
objective lens diameter. The higher the result the better the
The old adage “you get
what you pay for” is never truer than when applied to binoculars. If
you skimp on price you most definitely know it (and regret it)
sooner than later.
The distance between
your eye and the eyepiece is referred to as "eye relief". Good eye
relief is one of the important performance factors. Eye relief is
especially important if you wear glasses. If you need to use
binoculars while wearing spectacles or sunglasses look for models
that provide at least 15mm of eye relief.
The objective lenses
are found at the front of the binoculars. They gather light from
whatever it is you’re looking at, and magnify the resulting image.
This image at first, is upside down, and needs to be righted. This
is the function of the prisms; they correct the orientation of the
image so that by the time it gets to your eye it’s focused the right
Types of Prisms
Porro Prisms Models--Porro
prism models will give you the best optics for your dollar, but lack
the durability and compactness of roof prism models. You can easily
recognize them by their offset tubes--the objective lens and the
occular lens are not in line. The porro design is generally
optically superior to the roof prism design, especially when dealing
with mid-priced binoculars. Porro prism models are easier to adjust
for the spacing between your eyes because they have a single pivot
between the two lens tubes of the binoculars.
Sub-Types--There's something else you should take into consideration
if you are going to buy a Porro prism model.
Model--They are the best prism model type. They are designed with
superior optical glass that results in clearer images. This type is
what you should aim for.
Models--BK-7 prisms are usually found in lower priced models. They
produce an inferior image and some manufacturers will not include
this particular technical detail under the specification list
because those of you in the know will be alerted to the fact this is
an inferior design.
Roof Prism Models--are
more compact. This is possible because of the way they are
designed...they have straight tubes because the objective (front)
lens is in line with the ocular (rear) lens. They commonly have two
pivot points between the tubes, thus are harder to adjust for the
spacing between your eyes. Roof prism models are usually more
durable and compact, but you'll need to spend more money to get good
optical quality (or the equivalent optical quality of a Porro prism
model with the same technical specifications).
through your binoculars is influenced by the use and type of
anti-reflective coating on the binoculars lenses. How come? Because
when anti-reflective coating is applied to the glass surface of
binoculars it increases the amount of light transmitted to your eye.
In simple terms what this means is the better the type of
anti-reflective coating the sharper the resultant image.
Here are some of the
terms you may come across associated with lens anti-reflective
C: Some surfaces
FC: Other than plastic lenses, all surfaces coated
MC: Some surfaces coated with multiple layers
FMC: All glass surfaces coated with multiple layers
FMC--Fully Multi-Coated means that all air-to-glass surfaces have
been multiple-layer coated which translates into the best image.
This is the best quality, the downside being they are going to cost
For The Safari
Traveller This Is What You Should Remember
Best avoid zoom
binoculars. The theory seems appealing but the reality is very
different. Most zoom models are virtually impossible to hold steady
at higher magnifications and have very limited fields-of-view
Compact Models. Since
you're going to be on the move a lot and most likely doing most of
your viewing in good daylight, you should probably settle for a good
compact model--models with a magnification between 7 and 10.
Probably the best all-rounder is a good pair of 7x35 model. They are
bright enough to allow observation in low light conditions and
portable enough not to be a weight burden.
Getting a pair of
binoculars is a lifetime investment so try and buy the best possible
model that you can. Remember, you get what you pay for!
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