1. ## exit pupil

Sitting here with a pair of 25X100 binos on a trpiod and a pair of 11x70
binos on a tripod, and looking at them from a couple of feet away (call it
70 cm) I see that the 25x100s have an eyepiece lens 26mm in diameter and
the smaller pair has an eyepiece lens 22.5 mm in diameter. From this
distance I see an illuminated circle (both are focused on the same object)
about 4mm in the larger pair and about 8mm in diameter in the smaller
pair. Is this the same phenomenon I've heard called "exit pupil" ? Are
these numbers about what one might expect ?

The larger instrument's aperture/magnification ratio is 4 while the
smaller instrument's ratio is 6.3. I would have expected a noticable
difference in image brightness, with the smaller instrument giving the
brighter image. Is it credible that the fact that I don't see a
difference is due to my aging eyes not being able to use the 8mm exit
pupil (if that is in fact what I measured) ?

Or am I completely at sea ?

2. ## exit pupil

On Mon, 22 May 2006 18:36:14 -0500, Tom Rauschenbach wrote
(in article <pan.2006.05.22.23.36.12.389931@tomsdomain.org>) :

Yes, the exit pupil will be given by the Objective diameter divided by the
magnification. The diameter of the eyepiece lens is generally unrelated to
the exit pupil.

If all other things are equal (lens coatings, prism loss, etc) then the
instrument with the larger exit pupil will provide a brighter image for
non-stellar objects. That doesn't necessarily mean you will see as much
through it because magnification also plays a big role.

If you are doing this in daylight then your pupils will be significantly
smaller than your maximum. You are definitely not sing the full exit pupil.
The fact that you eyes are well back from the "eye point" of the instrument
will also be a factor although I'm not sure exactly how that would play out.

No, I think you are well on land <g>.

--
Bill Tschumy
Think Astronomy -- Austin, TX
http://www.thinkastronomy.com

3. ## exit pupil

On Mon, 22 May 2006 19:36:14 -0400, Tom Rauschenbach
<tomsusenet@tomsdomain.org> wrote:

Yes to the first question.

For the second question -- a qualified yes: The diameter of the exit
pupil will change when the focus is changed. The difference is
negligible for a telescope; but for some binoculars, particularly
lower powered binoculars, it can be quite noticeable -- and
measurable. Therefor (for astronomical purposes) you should focus the
binoculars on the most distant object in sight when measuring exit
pupil diameters.

This is a confusing realm for most people -- even for some of the
self-proclaimed 'experts'. For example (assuming the object being
observed fits wholly within both binocular fields of view):

One person might point the two pair of binoculars at the same extended
celestial object and proclaim: "The 100mm binoculars show greater
image brightness." This person's reasoning is base on a comparison of
the total amount of light concentrated into the object's ENTIRE image.

Another person might perform the identical experiment and proclaim:
"The 70mm binoculars show greater image brightness." This person's
reasoning is based on the total amount of light concentrated within
equal "apparent size areas" within the images of the object. (By
"apparent size", angular extent on the background sky is NOT being
referred to. Instead, equal apparent angular extents as seen by the
observer's eye while looking through the binoculars is being referred
to).

Within their contexts, each of the above individuals would be correct!

Yet, in the real world, observing under a starry sky, all one need do
is take a glance at any extended, nebulous, celestial object that will
entirely fit within the higher powered binoculars' field. It will
become quite obvious which binocular is better for observing such
objects. (Aging eyes will not change the outcome of this experiment).

Numbers and mathematics, even in the absence of any and all
mathematical errors can and do mislead many people -- not just amateur
astronomers When in doubt, do a real world experiment!

Willie,
So long and
Thanks for all the fish!