A 77mm objective lens has around 65% larger light gathering area than a 60mm
lens (77^2 / 60^2 = 1.6469444...) However, the human retina has a non-linear
response to light, so 65% more light does not mean a 65% "brighter" image -
particularly since the human eye has it's own auto-exposure system (i.e. the
pupil.)
Where the objective lens size does make a difference is in at the extremes of
the eye's capability. A large objective lens may produce
an image too bright for the human eye in strong lighting conditions, but also
produces a brighter image in dull lighting conditions (which is arguably the
more likely scenario.)
Naturally, there is a weight penalty with larger scopes that should not be
underestimated - a 50mm scope that you can happily carry everywhere will
outperform the 83mm cumbersome giant left at home because you "didn't need it"
- and of course did.
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