Aging Eyes and Pupil Size

Nothing is more impressive than a low-power, wide-field view of the Milky Way. There is, however, a personal lower limit on the magnification that should be used on any given instrument. It depends on the size of your eye's pupil, and that gets smaller as you get older.

The usual recommendation for the lowest magnification is 3.5x per inch (1.4x per cm) of aperture; this is so the exit pupil of the instrument will not exceed 7mm, the generally accepted diameter of a dark-adapted, fully-expanded pupil of a human eye.

There is considerable disagreement on the subject, even among experts in the field. A very popular and respected author says that an exit pupil larger than the eye's pupil means that some of the instrument's light collecting power is wasted. Furthermore, he contends that an exit pupil of about one millimeter smaller than the eye's is preferred for astronomy.

On the other hand, a prominent manufacturer of small instruments and eyepieces states that this is but one of many telescope myths. It is of little or no concern for refractors, since both image brightness and resolution are as large as possible at that magnification. For reflectors, larger exit pupils do waste light because the black spot in the exit pupil becomes larger. This black spot is caused by the obstructing secondary mirror. He claims that a reflector's low-power limit is reached not by the size of the exit pupil but only when this black spot becomes obtrusive.

Traditional "night glasses" (7X50 binoculars) are called that because they yield a 7mm exit pupil (50mm aperture/7 power.) Depending on whether you believe it fact or myth, they are perfect for the average dark-adapted eye. In defense of it being fact, I have never heard of a pair of binoculars with an exit pupil larger than 7mm.

In any age group there is considerable variation in pupil size. For example, at the peak age of 15, the dark-adapted pupil can vary from 5mm to 9mm with different individuals. After 25 years of age the average pupil size steadily decreases, though not at a steady rate. An examination of the accompanying table shows that it takes only 5 years, to age 30, to drop 0.5mm, yet it takes 20 years, from 60 to 80, to decrease that 0.5mm. The rate of shrinking of the pupil obviously slows down. This unavoidable reduction in aperture demands that visual observers match the exit pupil of their telescopes to the size of their dark-adapted pupil.

The table is set up to show the age at which you can expect to have a given pupil size from 7.0mm to 4.5mm. The last two columns give the factors for the lowest useful magnification for that pupil size, if indeed a larger exit pupil offers no gain in starlight. A quick calculation shows that an average 20 year old could go as low as 28x on an 8" (20cm) scope, whereas the average 80 year old would be wasting precious light outside the pupil if she/he went much below 43x.

Remember, these ages versus pupil sizes are only averages. The next time you are observing at a dark site, take along a millimeter rule and get a friend to measure your pupil diameter, then choose the magnification that will optimize the exit pupil at your eyepiece.

Here's hoping that you'll be pleasantly surprised with a larger-than-average-for-your-age pupil size!

Rob Roy

Hamilton Amateur Astronomers
Maintained by Rob Roy