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meteoptic-l [ät] ursa.fi
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Whoa! The f/ratio of an optical instrument has _no_ bearing on image
brightness when used visually! That's amateur-astronomy myth #1, and like
other myths, is false. Images coming through the eyepieces of telescopes
and binoculars are _parallel_light_ (or nearly so), so image brightness
depends solely on the ratio of the objective diameter to the exit pupil
diameter. Find a pair of binoculars where the f/ratio is given---and
given
the light paths of some types, does f/ratio have any meaning at all in
such optics!?
\Brian
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