High-Power Effects

At the other end of the scale, there is such a thing as too small an exit pupil. The basic limit is imposed by the diffraction inherent in any telescope's aperture. Use a magnification of more than 50x per inch of aperture, even in perfect atmospheric seeing, and you're just magnifying diffraction fuzz. This means the minimum useful pupil size for any telescope is 0.5 mm.

But even with a 1-mm pupil you're likely to notice some bothersome effects. You may see your retina's blood vessels superimposed on Jupiter, along with "floaters" — microscopic specks and strings of junk in your eyeball fluid. Floaters tend to increase with age. Other fine irregularities may be visible too.

Normally the neural network behind your retina does a remarkable job of image-processing all these things out of view. However, it tends to get thrown for a loop when the image is formed by narrow light cones from a tiny pupil. This is only logical. The eye never uses a 1-mm pupil in nature, so our visual processing system didn't evolve to fix whatever problems such a small pupil presents. If you keep glimpsing floaters, graininess, and blood vessels at very high power, ignore them.

A more pressing problem with small pupils is the issue of eye relief. This is how far the exit pupil floats behind the glass of the eyepiece. The amount of eye relief depends greatly on the eyepiece design. However, the smaller the exit pupil the closer to the glass it will probably be.

Since you have to get the exit pupil into your eye, you may have to crowd in mighty close. If you need to wear glasses while observing (to correct for astigmatism, say), you may not be able to get your eyes close enough to see the whole field of view. Check this before buying binoculars.

On a telescope, you can obtain good eye relief at high power by using a moderately long-focus eyepiece with a Barlow lens. Barlows once had a poor reputation, but modern, well-designed ones with multicoated lens elements do not degrade the image significantly. In my 12.5-inch, I really can't see the difference between the 180x view in a 10.5-mm eyepiece and the 180x view in a 26-mm eyepiece of the same design with a 2.5x Barlow — except that the latter gives more comfortable eye relief.

When all is said and done, you'll probably end up most pleased with a telescope's views when its exit pupil is from 2 to 5 mm across. By no coincidence, this is just about the everyday working size of the pupils that nature gave you.