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Message: <blockquote data-quote="ronh" data-source="post: 1688124" data-attributes="member: 55514">Matt,On the average, a young adult's pupils will dilate to 7mm in the dark, but this value shrinks with age. Heres a plot of average values. <a href="http://calgary.rasc.ca/telescopes/pupil_size.gif" target="_blank">http://calgary.rasc.ca/telescopes/pupil_size.gif</a>But the scatter about the average is quite large. Many people deviate from the average values by as much as 1mm in either direction, some even more. So when buying astronomical binos, it is nice to know your own eye's measurements. In daytime, however, the aging and individual effects pretty much disappear. In broad daylight, 2.5mm is a good assumption. In about "average twilight", around 4-5mm,  and as twilight deepens, a 7x42 or even 7x50 works best, provided your eyes can open up to accept most of the light of the 6 or 7mm exit pupil.I agree with Rick that it is very quick and easy to get the view when the exit pupil is considerably larger than the eye. In addition there is a theoretical advantage: the small eye pupil effectively stops the aperture down, resulting in a larger effective focal ratio, and better optical performance than at the actual f/3.5-f/4.  Methods of determining one's pupil diameter are tedious and error prone, except for one. Hold a ruler right under your eye, and in the lighting conditions of interest, snap a closeup photo. Which if its dark and the flash goes off will of course blind the hell out of you, but it's all for the good of science, you know.I only wish I had had the cash, and taste, for an 8x56 Zeiss while I was in college! By age 21, I at least had myself a "coated" Swift Aerolite 7x50 ($50, or 25 pitchers of beer) pretty disciplined, huh? Ron</blockquote>

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