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This answer to Not Able to View Objects with Barlow Lens includes a calculation of exit pupil diameter as a way to address the limits of useful magnification when observing planets with a small telescope. A comment there cites Sky & Telescope's Stargazer's Corner: Adventures Under the Night Sky's Simple Formulas for the Telescope Owner for the math.

EXIT PUPIL

The exit pupil is the diameter of the "light pencil" that emerges from the eyepiece. The pupil of fully dark-adapted human eye can dilate to about 7mm diameter, so an exit pupil in excess of 7mm is passing more light than the eye can accept. On the other hand, as the exit pupil decreases below 7mm, lack of light becomes the basic limiting factor to what you can see at night. Exit pupils of less than about 0.5mm are so small and pass so little light to the eye that they are functionally useless. Actually, I like exit pupils of at least 1.0mm for decent viewing.

Exit pupil can be calculated by dividing the telescope's clear aperture (in millimeters) by the magnification produced by the ocular in use. Thus:

APERTURE / MAGNIFICATION = EXIT PUPIL

For example, our 100mm clear aperture telescope with a 10mm ocular is operating at 100x magnification and therefore has a 1.0mm exit pupil (100/100=1)

Another way to calculate exit pupil is to divide the eyepiece focal length in millimeters by the telescope's focal ratio (f/stop).

OCULAR FOCAL LENGTH / TELESCOPE FOCAL RATIO = EXIT PUPIL

Thus, a 10mm ocular in our f/10 (100mm clear aperture and 1000mm focal length) telescope has a 1.0mm exit pupil (10/10=1). Either formula results in the same answer.

I've always paid attention to eye relief of the exit pupil, which is how far it falls beyond the eyepiece. My reference point here are my binoculars which have large diameter eyepiece optics which allows them to have a long eye relief; passing them back and forth between an eyeglass-corrected vision person and someone without glasses is just a quick rotation of the eyecups to different distances.

But I never looked at the diameter of the exit pupil, as it seems not to be an independently adjustable parameter of the optics; the exit pupil diameter is "baked in" to the choice of objective aperture and desired magnification.

As a side note it's worth mentioning that our eye's entrance pupil diameter is often cited around 6 mm or 7 mm (in the block quote) so anything larger than that is wasted. That was the key behind the answers to:

Question: How does eyepiece exit pupil diameter affect image clarity and viewing experience?

The block quote relates small exit pupil diameters to "so little light to the eye that they are functionally useless." That might apply to extended objects differently than it would to unresolved objects like stars, asteroids or even distant planets in modest telescopes.

Astronomical seeing and diffraction may also come into play; these effects can spread even unresolved objects in to apparently extended objects.

Related:


Nikon Monarch 42 mm binoculars (for which I have some familiarity but no affiliation)

Magnification 10x 8x
Objective Diameter (mm) 42 42
Angular Field of View (Real) 5.5° 6.3°
Angular Field of View (Apparent) 51.3° 47.5°
Exit Pupil (mm) 4.2 5.25
Eye Relief (mm) 18.4 19.5

Borrowed from How far are space-walking helmet faceplates from the eyes? How far will I have to hold my "space binoculars" from my eyes during a space walk? :

screenshot from How to Adjust Your Binoculars (Presented by Nikon Canada) screenshot from Understanding Binoculars: Eye Relief

See also Nikon Monarch Binocular Eyecup Repair How-to DIY

What it looks like when you are too far away from the eyepiece. From Wikipedia's exit pupil. If you were looking for something in a field of stars this "tunnel vision" would make it a lot more difficult.

loss of apparent field of view when viewing beyond the exit pupil

Cropped and annotated from here

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Exit pupil diameter is the determining factor of the apparent surface brightness (that is, brightness per unit area) of extended objects through a telescope or binoculars.

A 100,000,000,000,000,000mm aperture telescope operating a 2mm exit pupil will show an extended object at exactly the same brightness per unit area as a 100mm aperture telescope operating at a 2mm exit pupil. The difference of course, would be in magnification.

As exit pupil decreases in diameter, brightness decreases by the square (just like the telescope's aperture). A 2mm exit pupil contains 4x more light than a 1mm exit pupil.

The largest useful exit pupil is one that matches the observer's own dilated pupil. The smallest useful exit pupil is subjective and depends on the target. In some cases, an exit pupil smaller than 5mm may be too dim. In other cases, an exit pupil as small as 0.25mm is still acceptable.

When exit pupil gets small, however, floaters and other contaminants in the eye become very large proportional to the size of the pupil, and thus become more readily visible. When the exit pupil gets very small, there is so little light entering the eye that contrast perception suffers.

Exit pupil does NOT affect the perceived brightness of stars (at least not until the Airy pattern starts to become large enough for its light to be spread out). The light concentrated in the Airy pattern remains more or less constant in brightness regardless of how small the exit pupil gets (however, it will get dimmer once exit pupil starts to exceed your eye's entrance pupil).

For this reason, using higher magnifcation and smaller exit pupils can make star clusters appear more prominent, and will let you see fainter stars. If a telescope is sufficient aperture to barely see Pluto, the best chance of seeing Pluto will come from very high magnification, which will drop the exit pupil and dim the background skyglow, without making the ~15th magnitude speck of light dimmmer. The increase in relative contrast will make it easier to see.

Exit pupil should always be thought of as the other side of the magnification/brightness coin. You should strive to find the optimal balance of magnification and exit pupil against specific targets to achieve an optimum visual perception of them. This will vary from target to target, vary depending on light pollution levels, and vary from individual observer to individual observer. A zoom eyepiece is a useful tool in finding this optimal balance.

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