| |
1.
What do you need?
For most
people, you use a binocular for general purposes: spectator sports,
sight seeing, etc. Except for some special activities, the most
important criteria you need in choosing a binocular is portability
and ease-of-use. If a salesperson tries to convince you otherwise,
walk away.
The
first spec we need to decide is power of magnification. For all
around purposes, 5x to 10x pretty much covers everything. It is
a common mistake to assume that the higher the power, the more "powerful"
your binocular is. The fact of the matter is you will be exhausted
in a few seconds looking at the "dancing" image through
a 15x binocular, unless you want to carry a tripod everywhere you
go. Some opera glasses offer 2x and 3x power, but you definitely
will not regret it if you take a regular 8x one with you instead.
Another advice: never buy a zoom binocular! You may think it is
useful to change the power of magnification, but it's cumbersome,
difficult to use, and easily broken.
The
second number is the size of the objective lens. In a 8x21 binocular,
the objective lens diameter is 21 mm. It is true that the larger
the objective lens you have, the more light you gather from the
object you are viewing. However, a larger objective lens also means
a longer and heavy binocular body. So, you need to balance portability
and lens size. I have boxes of binocular samples from manufacturers,
but I find myself usually put a 8x21 or a 10x25 compact binocular
in my pocket before leaving home.
There is also a practical limit on the size of the objective lens.
This is determined by the exit pupil of a binocular. For a 8x40
binocular, divide 40 mm (objective lens size) by 8 (power of magnification)
gives 5 mm, which is the size of the exit pupil. If you want to
take full advantage of a large objective lens, i.e., to collect
all the light funneled through the binocular, the binocular's exit
pupil cannot be larger than your eye's entrance pupil (you know,
the black hole at the center of your eyeball). This is where the
limit comes in. The entrance pupil of a human eye changes with the
light condition, but the maximum size is limited by age. The entrance
pupil can reach 8 mm for a teenage, but is only ~4 mm for a 50 year
old. For an entrance pupil of 5 mm, a 7x35 is just as good as a
7x50, but the latter is much heavier.
Of
course, there are special needs. For example, experienced bird watchers
prefer a 40 or 42 mm objective lens to gather more light and see
more details in deep shadow. A mariner likes 7x50 because it is
more forgiving when the deck is constantly moving.
2.
Where to buy
Most people
buy their binoculars from camera stores, discount department stores,
and sports/hunting stores. The truth is that none of these stores
is specialized in binoculars and the salesperson usually knows nothing
about binoculars, except what is printed on the box. It is ok to
buy from these places but you should be prepared to do your own
quality check and know what you need. Some mail order firms tend
to be specialized in binoculars and they usually carry most common
brand names. But make sure they have a return policy, because you
cannot see what you are buying on the phone. A good mail order business
should allow a 30 day trial period.
Here
is a common myth about brand names. Many people tend to believe
that German brands and now some Japanese brands offer the best quality.
But, is it worth 10 times the price just for the name? Here is some
news you may find shocking: recently I visited several large optics
factories in China and found that these factories make most of the
common brand names we know. As a matter of fact, these different
brand names often come from the same production line. The only difference
is the label! Unless you want to pay $800 for a binocular with the
ultimate quality, the difference between common brands is negligible.
You can usually pay less than $100 for a premium quality binocular.
The
factories supply exporters, who in turn sell to different brand
name distributors, who then supply wholesalers, who supply retailers,
who finally sell to consumers...... You got the picture: between
you and the manufacturer, there are five or more middle men who
may stick different labels on the box plus 4-10 times the original
price.
How
to get the best out of your money? Thanks to the WEB, many factories
now are interested in having their presence on the Net. You can
buy premium optical instruments directly from the factory. After
all, the internet signals the fall of all walls and barriers. Why
do we need all those middle men? Click on compact
binoculars or standard binoculars, I'll show you how to directly
order from the factory.
3.
Quality check
When
you have a binocular in your hands, you can do a few simple tests
to determine the quality. Remember: do not be fooled by the external
appearance or the high price tag. A good binocular should pass several
basic tests:
(1)
Collimation: collimation refers to the alignment of the optical
axis. The best performance is reached when the optical axis of the
objective lens passes through the optical center of all lenses and
prisms. This should hold true for both optical barrel assemblies.
For most binoculars, poor collimation is not immediately obvious
when you first pick it up and view through it. This is because your
eyeball will adjust to compensate for misalignment in the optical
instrument. Of course, if you let your eye muscle strain like this
for a few minutes, you will feel the pain soon. Here is a simple
way to check for collimation. First, look at a target ~100 yards
away with the binocular. Be sure to adjust the center focus as well
as the right eye piece. Now, relax your eye for a minute by taking
the binocular down. Look at the target again with the left objective
lens blocked by your hand. After a minute, quick;y remove your hand
which covers the objective lens. If the target appears to be out
of focus immediately but becomes in focus after a second or two,
the binocular is not collimated. Your eye is doing the hard work
to adjust collimation and focus. Choose a different binocular.
(2)
Resolution: resolution determines how sharp the image is and how
much fine detail you can see. You can usually check this by comparing
a few different binoculars and look at the same target ~100 yards
away. The manufacturer usually specifies resolution by a number,
e.g., 5 seconds of arc. This means that the binocular should be
able to resolve two fine features at an angle of 5" apart.
Professionals use standard test charts very much like the one used
in an eye doctor's office. You usually do not need to bother with
this kind of tests, unless you are talking about a $1000 instrument.
These test charts are available in the references I cited earlier.
But, your common sense is sufficient for most purposes.
(3)
Star test: the ultimate test of an optical instrument is the so
called "star test". This is done by looking at the image
of a distant star in the night sky. To do this, locate a bright
star at the center of the field-of-view. The perfect image should
appear as a round disk, with a few weak, concentric rings due to
diffraction. Almost all binoculars will show some deviation from
this perfect image. Some asymmetry is acceptable, but if your binocular's
star test yields an image which is something like a triangle or
a stick, try to get ride of it.
|
