Generally, the classical advice is to stay with at least a 4 mm exit pupil and 5 is better, but with today's high magnification astronomy binoculars, such as 30x80s, 22 x70s, 16x50 and so on, lot's of 3mm or so exit pupils binos being used. Probably more important to stay with a large objective in the neighborhood of 50 mm and up to get that 3 mm exit pupil, though, for the sake of resolution.
A 16x50 would be a fine choice for observing bright, small to mid-size open clusters, splitting double star and a great choice for observing the moon (when placed on a tripod). it would be at a disadvantage for dimmer objects such as faint nebulae and galaxies. For a more general purpose astronomy binocular, a 5mm exit pupil is better, yes, but a 16x50 with it's 3+ exit pupil is still useful.
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