wrote:
>
> 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.)
There is also the point that after you are about 26 (I think) the amount
of light you need doubles every 13 years - so I need about 8 times as
much as I used to - that's heading for 180mm objective! And the back
gets older too!
Brian Fleming
Melbourne
Birding-Aus is on the Web at
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