Is it possible, with better than normal eyesight, to see the crescent of Venus?
That question has been controversial, but in fact some people can. The rough rule of thumb is that someone with excellent vision can just resolve two image elements 60 arcseconds (60") apart. At times, this is enough resolution to make out the crescent.
To reduce glare, examine Venus in bright twilight rather than in darkness. You can improve your chances by looking through a clean, round hole 2 or 3 millimeters in diameter held just in front of your eye. This reduces spikes and blurring due to optical aberrations away from the center of your pupil. Experiment with different-size holes, as Fred Schaaf urged in the November 2002 issue, page 104.
— Alan MacRobert