Questions About Binoculars and Spotting Scopes
What do the numbers mean on binoculars and spotting
This is the most common question about binoculars and it is a
good one. You will see combinations of numbers like 7x35, 8x42,
10x50 or 7-15x35 and many more. The numbers to the left of the "x"
always refer to how much magnification the binocular has. The number
to the right of the "x" indicate how big the lens is at the bottom
of the binocular (this is called the objective lens).
So, 7x35 means this binocular magnifies objects so they appear 7
times closer. This binocular also has a 35mm diameter "objective
lens". A 10x50 binocular magnifies objects so they appear 10 times
closer and has an "objective lens" 50mm in diameter. The 7-15x35
binocular is a zoom model because you can change the magnification
from 7 times to 15 times. This binocular has a 35mm "objective
Should I buy a binocular with the highest
Not necessarily. Here are a few things to consider when selecting
a binocular to determine what magnification will be best for you.
Generally, the higher the magnification the more difficult it
will be to have a bright, steady and wide image. Why is
Try thinking of magnification in terms of using your
portable video camera.
What happens when you
"zoom" in on objects (using the highest
You will find that several things happen:
- The image is "jumpy" (it is difficult to hold the camera
- You only see a very small "field of view" or viewing area
- The image is not as sharp and clear
- The image is not as bright
Generally speaking when you increase magnification you may
increase the possibility of all the above results. Most binoculars
sold today range from 7x to 10x. Zoom models offer a range of
magnifications from 7x to 20x or even higher.
Why is brightness important and how do I determine
how bright a binocular is?
The brighter the image when looking through a binocular or
spotting scope the better chance you will have of seeing a sharp and
detailed image of objects viewed.
Brightness if affected by:
- How big the "exit pupil" is of the binocular. (this is an easy
thing to determine and is explained below).
- The types and number of "coatings" applied to the optical
What is an "Exit Pupil"?
The exit pupil is the circle of light you see when holding a
binocular or spotting scope about 6 inches away from your eyes and
looking at the surface of the eye pieces. This little bright circle
is the amount of light reaching your eyes.
The size of the exit pupil is easy to calculate. You simply
divide the magnification of the binocular into the diameter of the
"objective lens" or bottom lens of the binocular or spotting scope.
So, a 7x35 binocular has an exit pupil diameter of 5mm (35 divided
by 7). A 10x50 binocular also has an exit pupil of 5mm (50 divided
Why is "Exit Pupil" important?
You can use the diameter of the exit pupil calculation to
determine how much relative light is available under different
conditions. When we are in bright light our own eye pupil constricts
to as small as 1mm in diameter. However, in very low light such as
in the afternoon or early morning our eye pupil can expand to around
7mm. Our pupils open and close to allow enough light to see details
and a sharp image.
The "exit pupil" on a binocular can be used to correspond to our
own eye pupil. Generally a 5mm "exit pupil" will provide good
brightness over many conditions. But in low light conditions it will
not provide enough light to see as your own eye pupil may want. The
result is a darker image and you may not see as much detail.
What are lens coatings?
Coatings are applied to the glass elements to reduce the amount
of light with is lost due to reflections. Generally binoculars and
spotting scopes are coated with a single layer of coating applied to
each glass surface. More expensive models have multi-coatings
applied with further reduces the amount of light lost to
What is "wide angle"?
Binoculars typically come in either standard or wide angle
designs. Wide angle designs provide a wider field of view than
standard designs. Field of view is simply the area which you can see
at a certain distance. The image viewed is usually measured in the
number of feet you can see across at a distance of 1000 yards. A
binocular showing a field of view of 430' at 1000 yds. means you can
see a horizontal distance of 430 feet when looking out to a distance
of 1000 yards.
What is "long eye relief" and why is it
Eye relief is the distance from the surface of the eye piece lens
where you can see the full field of view of the image you are
looking at. Most binoculars have a standard eye relief distance of
from 9mm to 12mm. This is long enough for just about everybody
unless you wear eye glasses and you don't feel like taking your
glasses off to see through the binocular!
Long eye relief is primarily a characteristic which specifically
benefits people who wear eye glasses. But it is also great for
everyone else because it is great to use them when wearing sun
glasses or eye glasses! Some binoculars with long eye relief do not
have a "wide angle" field of view. This is because when designing
optical systems it is difficult to provide both long eye relief and
wide angle in the same system. But several of our Alpen Pro
binoculars offer both long eye relief and wide angle field of
What is a "fixed focus" binocular?
These binoculars are great because they don't have the
traditional focus wheel or eyepiece focus adjustment. They are great
for action sports viewing or viewing situations where you don't want
to loose time having to adjust your binocular. These binoculars are
designed so that you can see objects from about 70 feet away and
beyond. You just pick them up and look! Alpen offers both a standard
and long eye relief design. The long eye relief model is great if
you wear glasses!
What is the difference between a "porro prism"
binocular and a "roof prism" binocular?
Porro prism binoculars have the "offset" design. The eye pieces
are not the same distance apart at the objective lenses. Porro prism
binoculars offer great viewing performance, are more "traditional"
and are generally less expensive to manufacture.
"Roof prism" binoculars are distinguished by their "in-line"
appearance. The eye pieces and objective lenses share the same axis.
These binoculars also offer great viewing, are less bulky, and
generally more expensive to manufacture.
Both prism designs can provide great viewing and it is generally
just personal preference that determines which one is best for
What is different about compact
Compact binoculars are developed to provide a small, lighter
weight binocular which can be more easily carried. The advantages of
compact binoculars is their small size, convenient use and they are
great when you want to transport them with you when hiking or when
larger binoculars are just not convenient to use.
But compact binoculars generally do not offer the brightness that
larger binoculars do. Remember the discussion above about "exit
pupil"? Compact binoculars generally do not have large exit pupils
which means in low light conditions you will not see a bright or
sharp image. For example, a 10x25 compact binocular has an exit
pupil of only 2.5mm. During the day this is not a problem. But for
early morning or late afternoon viewing they will not provide as
much light as a larger standard design binocular.
How do I decide which binocular is best for
Buying a binocular involves "trade-offs". In order to get the
brightest possible image in low light conditions you will need low
magnification with a large "objective lens". But many of us don't
want to sacrifice magnification. After all isn't this why we are
buying a binocular? We want to see objects far away as though they
were close! You may also want a "wide field of view" so you can see
as much action as possible at a sporting event. So, you will find
that you need to know how, when and where you are most likely to use
your binocular. In the end you may decide to buy several different
models to meet the needs of the different types of viewing
situations you intend to enjoy.