Middle age and the telescope
Middle age and the telescope: One unfortunate consequence of aging is a reduction
in our eyes’ ability to dilate in the dark. By the time middle age arrives, our
pupils may dilate to as little as 5mm, instead of the 7mm of our youth. Nearly one-half
the light of the 7mm exit pupil of a long focal length eyepiece will fall on a middle-aged
observer’s iris, rather than entering the 5mm pupil of his or her eye, and consequently
will be wasted.
If you’re middle-aged, then, buy very low power eyepieces primarily for their wider
fields of view. Don’t buy them for brighter images if your eyes can’t dilate to
match their exit pupil. Images are brightest when the eyepiece exit pupil matches
your eye’s pupil.