So, an 8x42 binocular (the
most commonly used configuration among birders) has an
exit pupil of about 5mm. That’s
going to be a good amount of
light coming into your eyes, even
at dawn or dusk. Now take that
same 42mm objective lens, but
increase the power to 10x and
you’ll see that even though we still
have the same size front lens, the
exit pupil drops to 4mm, resulting
in less light getting to your eyes.
An ideal birding binocular will
have an exit pupil of 4 to 5mm for
good performance in low light.
This is exactly why 8x32s, 8x42s,
10x42s, and, to a lesser extent,
10x50s make up the majority of
binoculars used by birders.
It’s never easy making choices,
knowing that obtaining one thing
requires giving up another. The
sting of not being able to have it
all can be softened a bit when we
make our decisions knowing what
we are giving up in exchange for
what we are acquiring. With binoculars, this is certainly the case.
If we could only be this fortunate
with all the choices presented
to us in life, it might be easier to
decide if it’s a good morning to
lace up the boots for a stroll in the
prairie or sleep in a little longer. a
Ben Lizdas has 16 years of
experience in the birding optics
business and is the ad sales director
for Bird Watcher’s Digest.
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