Exit Pupil

Greetings,

About 6 weeks ago I purchased a pair of Leupold 10x50 binocs. This means
that these binocs have an exit pupil of 5. Right? The problem with these is
the field of view is smaller than what I'd like.

I've been reading that as one gets older, the pupil doesn't dilate like it
used to. I'm 47.5 years of age. Have I reached that point?

I ask this because I'm considering buying some binocs(used) with a wider
fov. I'm considering 7x50, 8x40, 7x42 and 8x56( maybe you folks have better
suggestions).

Will the 7x50's and the 8x56's have too large of an exit pupil. My thinking
is for astronomy, the more light the better. Is this thinking correct?. How
will having too large of an exit pupil affect my astronomy observing? I
know the 7x50's and the 8x56's have an exit pupil of 7.1 and 7. Is this too
large an exit pupil?

Would I be better of with 7x35's instead of 7x50's? 8x40's instead of
8x56's? Help!

Mike

"Mike Jenkins" wrote in message
thlink.net...
About 6 weeks ago I purchased a pair of Leupold 10x50 binocs. This means
that these binocs have an exit pupil of 5. Right? The problem with these
is
the field of view is smaller than what I'd like.

I've been reading that as one gets older, the pupil doesn't dilate like
it
used to. I'm 47.5 years of age. Have I reached that point?

I ask this because I'm considering buying some binocs(used) with a wider
fov. I'm considering 7x50, 8x40, 7x42 and 8x56( maybe you folks have
better
suggestions).

Will the 7x50's and the 8x56's have too large of an exit pupil. My
thinking
is for astronomy, the more light the better. Is this thinking correct?.
How
will having too large of an exit pupil affect my astronomy observing? I
know the 7x50's and the 8x56's have an exit pupil of 7.1 and 7. Is this
too
large an exit pupil?

Would I be better of with 7x35's instead of 7x50's? 8x40's instead of
8x56's? Help!

Mike,

You can measure your exit pupil when you are dark adapted. Make a long
strip of light cardboard (a manila folder works well) that tapers from 2mm
wide to 8mm wide. While looking at a star, place the narrow end of the stip
in front of your eye, as close to your pupil as possible. You should see
the star on each side of the stip. Slowly slide it so the thickness is
increasing. When the star blinks out on both sides of the strip, that width
is your exit pupil. If the star vanishes from only one side, it is not
quite centered in front of your eye. Try again.

Having said that, you may need to get a pair of binoculars with a larger
than needed exit pupil to get a wide field of view. I bought a pair of
7x42s with an 8 degree field knowing I will probably not use the entire
aperture, and will never use it during the day. (They had other features I
could not find on anything smaller too.) Actually, if you look around,
there are some with a smaller exit pupil that have a reasonably wide field.
The Nikon Venurer 8x32s and the Swarovski 8x30 SLCs come to mind, and the
Nikon 8x32 Superior Es have a pretty wide field too, I believe.

Clear skies, Alan

"Mike Jenkins" wrote in message
thlink.net...
Greetings,

About 6 weeks ago I purchased a pair of Leupold 10x50 binocs. This means
that these binocs have an exit pupil of 5. Right? The problem with these
is
the field of view is smaller than what I'd like.

Yep.

Will the 7x50's and the 8x56's have too large of an exit pupil. My
thinking
is for astronomy, the more light the better. Is this thinking correct?.
How
will having too large of an exit pupil affect my astronomy observing? I
know the 7x50's and the 8x56's have an exit pupil of 7.1 and 7. Is this
too
large an exit pupil?


The more light that reaches your retina the better. Unfortunately, an exit
pupil that exceeds your eyes pupil doesn't do that, it actually will cause
contrast and brightness to suffer. That being said, its still a matter of
personal choice. Of course contrast is largely an issue with DSO's, many of
which are just plain unappealing in 50-60mm low-power binos.

Of course if your eyes are only dialating to 4-5mm, you are only able to use
50% or so of the light available from a 7mm exit pupil. (Which of course
means your going to be getting the equivalent of 7x35s out of your 7x50s.)

Anyways, good luck.

"Alan French" wrote in message . ..
You can measure your exit pupil when you are dark adapted. Make a long
strip of light cardboard (a manila folder works well) that tapers from 2mm
wide to 8mm wide. While looking at a star, place the narrow end of the stip
in front of your eye, as close to your pupil as possible. .... [big snip]

Great tip Alan. I'm going to try this since everyone's vision is
unique. Using a general relationship, Mike, at 47 years of age, may
have aways to go to get down under a 5mm pupil size:

D_exit_pupil = 7 mm exp(-0.5[A/100]^2])

Age D_ep
10 7.0
20 6.9
30 6.7
40 6.5
50 6.2
60 5.8
70 5.5
80 5.1
90 4.7
100 4.2

Schaefer, B.E., Telescopic Limiting Magnitudes, Pub. Astron. Soc. Pac.
102:212-229 (Feb. 1990)
citing prior work of:
Kadlecova, V., Peleska, M., and Vasko, A. Nature 182:1520 (1958)
Kumnick, L.S., J. Opt. Soc. Am. 34:319 (1954)

- Kurt

"George Kinney" wrote in message ...

The more light that reaches your retina the better. Unfortunately, an exit
pupil that exceeds your eyes pupil doesn't do that, it actually will cause
contrast and brightness to suffer.

No, I don't think so. I tried some experiments with a friend's 8x56
binoculars, using circular masks of various sizes to modify them
down to 8x52, 8x48, 8x44, and 8x40. As far as I could tell, the
view through the binoculars with the 44mm mask was *exactly* the
same as through the unmasked binoculars. That agrees with a
measurement of 5.5mm for my pupils derived from other methods.

In other words, for any given magnification, the only drawbacks
for having an exit pupil wider than your eye's pupil is that
the binoculars are unnecessarily heavy and bulky, and possibly
unnecessarily expensive as well. A significant advantage is
that they are more forgiving about eye placement; when the
exit pupil matches your own pupils precisely, you lose light
unless your eyes are *precisely* behind the centers of the
eyepieces, which can be hard to achieve.

A propos of field of view, the true FOV of most 10x50 binoculars
is 5 degrees, but there are also several highly respected
wide-angle models with FOVs of 6.5 degrees or wider, nearly
as wide as standard 7X binoculars.

Having said all that, I must admit that I love my 7x35 binoculars,
and use them fairly frequently in preference to my 10x50. But
they certainly can't see such faint objects -- not even close!

- Tony Flanders