Exit pupil and the eye

I find exit pupils kind of confusing.

Seems like you can project an image of what you are observing by positioning a piece of paper at the proper eye relief distance for the exit pupil.

You can have an exit pupil so large that your fully dilated eye can't take in all the light.

You can have an exit pupil so small that tiny stuff in your eye interferes with your view.

One optical expert on this forum has written that the only thing the size of the exit pupil affects is brightness.

I don't see how all that can be true together.

Can someone please explain how the exit pupil and the eye interact, with a word or two about what changes and why when the size of the exit pupil changes?

Thanks for any help,

Steve

That’s about right – think the diameter of the shaft of light coming into your scope (say a 80mm in diameter shaft for a ST-80) divided by the power (say using a 26mm EP to give you 16X). The “exit pupil” (the diameter of the shaft of light coming out of your EP) would then be 80/16 or 5mm. Some people have up to 9mm dark-adapted irises, and as we get older (or if there is too much light to become completely dark-adapted in the first place – like most “suburban” areas) our maximum iris diameters will be less. I’ve heard the new standard is about 5mm for light polluted areas and for older observers, but mileage will vary.

BTW – the 7mm “standard” was set during WW-II (and before) by many Navies in the world that came up with the almost universal 7mm maximum as the figure for an “average” young sailor’s maximum dark-adapted iris diameter who had to stand watch, hence why the 7X50s became a almost world-wide watch bimolecular standard. This was an extremely important issue before the invention of RADAR as visual spotting of the “other guys” first could be the difference between your swimming (and maybe becoming shark bait) or sailing to your next port. As a 50mm objective was considered the minimum aperture to see a ship at 26 miles (60 foot superstructure seen over the curve of the earth from another 60 foot superstructure) and 7X gave the right “average” pupil size for maximum light throughput at night. 7X50s were also light enough for the sailor to hold reasonably steady for the 2 to 4 hour session involved. My Dad worked on this among other things at the predecessor of the Navel Research Lab outside of Washington DC at the time and told me how they came up with these things - smile.

Hope this helps to explain it ...

FWIW - TM

> Seems like you can project an image of what you are
> observing by positioning a piece of paper at the proper
> eye relief distance for the exit pupil.

Actually this is not quite true. If you hold a paper
near the eyepiece at the eye relief point, what you
get is an image of the objective lens. If you move
the paper back and forth a little, you will see that
the size of the pattern gets larger if you move
inside or outside the eyerelief point (actually
defocussing the image of the objective lens).
This change in the size of the pattern relative to
the eyerelief point will be most noticable if you
point the scope a something uniform like a white wall.

The diameter of this pattern on the paper at the
eye relief point *is* the exit pupil. Basically
its the diameter of the objective lens as re-imaged
at the eyepiece.

If you move the paper much farther away than
the eyerelief distance, you will start to get
an image of the target. And with some re-focussing
you can get a sharp image of the target
(moon, etc.) on the paper.

> You can have an exit pupil so large that your
> fully dilated eye can't take in all the light.

Indeed. At very low powers the exit pupil of
the telescope will be larger than the eye pupil.
If you assume a maximum eye pupil of 5mm, then
powers below 5x per inch of aperture will
prevent all the light from entering the eye.

> You can have an exit pupil so small that tiny
> stuff in your eye interferes with your view.

True! Exit pupils smaller than about 1mm (or
magnifications about 25x per inch of aperture)
will start to have this problem. Around 0.5mm
exit pupil (50x per inch) its a very
noticable effect for many people.

Its because the light cone through the eye becomes
narrow and and starts to approach the size of junk
(sometimes called "floaters") in the eye.
So the junk can cutoff a noticable amount of light.
If a bit of junk cuts off say 10% of the light at
0.5mm exit pupil, it only cuts off about 0.1% (= 10%/10^2)
at 5mm exit pupil. So the effect goes up fast at small
exit pupils (and high powers).

> One optical expert on this forum has written
> that the only thing the size of the exit pupil
> affects is brightness.

This is true that brightness and exit pupil are
related, but the other effects are also important
at high and low powers.

Hope this helps a little....