Smaller exit pupils better, but vibration counteracts it to an extent
by John Houston
Most people's eye lenses are distorted enough to produce spiky-looking stars if the binocular is something like a 7x50. Stars look more pleasing with a 5mm exit pupil shown in 10x50 or equivalent binoculars. Also, since stars are not extended objects (they are point sources unless magnification is very high, like 200x) so increasing the power of a binocular helps increase the contrast between the sky background (which darkens as power increases) and the stars allowing you to gain as much a magnitude and see many more stars. However, vibration on increasing power can reduce the gain because the star's light is then spread over an actual area, rather than being concentrated in a point-source. Lastly, people who are older than 40 often have eyes which no longer have irises that open beyond 4-5mm so binoculars with larger exit pupils cause a loss of light. What you gain from lower power, all things being equal, is a larger field of view. Where that is worth exchanging for the greater penetrating power of a higher power binocular is up to you to decide.
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