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The size of a persons maximum pupil dilation (useable exit pupil) depends on a few things. Most important is age. A younger person <30 will have a greater pupil dilation than a person > 30 but < 50 and similarly that person would have a greater pupil dilation than someone >50. Other physical factors will also affect a person's maximum pupil dilation including smoker/non smoker, heavy drinker/non drinker, high blood pressure/low blood pressure etc etc. The greater a person's maximum pupil dilation, the greater the eyepiece exit pupil that will offer them comfortable pleasing views, with good contrast and importantly no secondary shadow in a newtonian, cassegrain, SCT or MCT. The other thing which affects the exit pupil parameters are the darkness of the skies. You can easily use a much larger exit pupil under dark skies than you can under marginal or heavily light polluted skies.
One thing will never change, the smaller the exit pupil the greater the contrast and the lower the aberrations introduced by a persons own eye, eg astigmatism etc. Hence, you are always better to err on the side of being conservative going with a smaller exit pupil, as opposed to a larger one. As a general rule of thumb, people in the 30 to 50 age bracket should not exceed an exit pupil of about 7mm under dark skies and about 6mm under any form of light polluted skies. I am 48 and use a 6.88mm exit pupil under dark skies with good views. Under less than dark skies the view starts to lose a little contrast and I would not like an exit pupil any larger. A person under 30 yrs of age could "possibly" go up to 1mm over those parameters. If you limit yourself to a maximum exit pupil between 6 and 7mm you should be very safe.
Clear Skies
John Bambury
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