Sky & Telescope - January 2024 - 73

by an " × " , such as 7×35, 10×50, or even
20×80. That first number is the magnification
- how much bigger something
will appear compared to when it's
viewed with your eyes alone. So, with
10× binoculars, a bird that's perched in
a tree 20 meters (66 feet) away will look
as if it's only 2 m distant. You'll be able
to see features that would be impossible
to perceive without the magnification
boost. Expressed using terms astronomers
prefer, magnification allows you to
resolve finer detail - and that's as helpful
for a distant star cluster as it is for a
backyard bird.
The second number in the set refers
to the diameter of the twin front lenses
(in millimeters) - the so-called objective
lenses, or aperture. The importance
of this spec is easy to appreciate since
when it comes to optics, bigger is better.
Larger lenses gather more light the
same way a bucket left outside gathers
more rain than a cup. So, binoculars
with 50-mm objectives drink up twice
as much light as 35-mm lenses, bearing
in mind that light-gathering is a
function of area. Putting it all together,
10×50 binoculars yield a magnification
of 10× and have 50-mm-diameter
objective lenses.
But when it comes to astronomy,
how do we know if one model will
be better than another? For example,
would 8×56s show us more than
10×50s? One way to compare different
models is to calculate a basic performance
rating by doing exactly what
the numbers suggest - multiplying the
second figure by the first. In this scheme
IT'S ALL IN THE NUMBERS The most
important specifications for a pair of binoculars
are its magnification and the size of its lightgathering
objective lenses. These figures are
printed near the right eyepiece in this example.
As explained in the text, they indicate that this
particular model magnifies 10×, has 50-mm
front lenses, and yields a field of view 6.5° wide.
(first proposed by Canadian observer
Roy L. Bishop), 8×56s would rate 448,
while 10×50s would score 500, indicating
slightly better performance despite
having smaller objective lenses. This
rating system gives you a rough feel for
how magnification and aperture interact,
and their impact on how different
binoculars work under the stars.
But can two numbers really tell the
whole story? Not entirely. Magnification
and aperture mostly define how
a given pair will perform, but (as with
most things in life) there are inevitable
tradeoffs to consider. For example,
20×80s unsurprisingly (with their
1,600 rating!) can show greater detail
than 10×50s. However, big binos are
CRUCIAL SPECS
On some binoculars,
such as this pair of
Nikon Aculons, the
magnification, objective-lens
diameter,
and field of view are
displayed between
the eyepieces.
a lot heavier and require a tripod to
work effectively. So, if portability and
ease-of-use are important to you - and
many would argue those are the main
strengths of binoculars - the lightweight
10×50s are a better choice. In
addition, the smaller pair will also show
you a bigger slice of the sky. The fact
of the matter is that (all other things
being equal) as magnification goes up,
the field of view goes down. Those hefty
20×80s will show you a circle of sky
roughly 2½° across - only one quarter
the area that 10×50s will deliver. That's
a big difference! It's true that things will
appear smaller, fainter, and less detailed
in the 10×50s, but you'll be able to take
in a much bigger swath of sky, which
makes locating your target far easier
than with the more powerful pair.
So, what about those other numbers
you sometimes see printed on binoculars?
It's not unusual to encounter
specifications such as " 367 feet at
1,000 yards, " for example. These figures
describe the diameter of the field of
view. To put it into English, the numbers
in this example tell us that if we
look at an office tower that's 1,000
yards away, you'll see 367 feet of its
height in your binoculars. Interesting,
but for astronomy, not terribly useful
since we don't measure the sky in feet
or yards. But you can convert to degrees
by simply dividing the first number in
the set by 52.4. Doing so reveals that
the binos we're discussing here have a
7°-wide field of view (367/52.4). Handy.
If the numbers are expressed in meters
instead of yards, you'd divide the first
figure by 17.5 instead. Thankfully, many
manufacturers simply give the field size
in degrees - no math required.
Of course, there's much more to
choosing binoculars than just the basic
specifications discussed here. Price,
brand, optical and mechanical quality,
plus a host of other factors all matter.
However, the first and most important
decision to make is to figure out which
combination of magnification and
aperture makes sense for the kind of
observing you like to do. And for that,
you really can begin your search with
just two numbers. ¢
sk yand tele scope .o r g * JANUARY 2024 73
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Sky & Telescope - January 2024

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