Are the F numbers in telescopes the same as for camera lens? As in 1.4 lets in twice as much light as 2.8? I got to wondering about that for focal reducers. Sure the field is wider but how 'bout the light?
F Stopped...........
Started by David W, 10/06/2002 02:44AM
Posted 10/06/2002 02:44AM
Opening Post
Posted 10/06/2002 03:56AM
#1
Yes, F numbers have the same meaning. But changing the
F on a scope is different from a camera. On scopes the
diameter is usually constant, but people change the F
with reducers / Barlows / etc. that alter the focal
length. On a camera chnaging the F usually means the
diameter changes (of course zoom lenses are more
complicated....).
On a scope, a focal reducer will reduce the F number.
Extended objects (including the sky background)
will appear brighter as a result.
By the way, F/1.4 lets in four times more light
than F/2.8 for given focal length.
F on a scope is different from a camera. On scopes the
diameter is usually constant, but people change the F
with reducers / Barlows / etc. that alter the focal
length. On a camera chnaging the F usually means the
diameter changes (of course zoom lenses are more
complicated....).
On a scope, a focal reducer will reduce the F number.
Extended objects (including the sky background)
will appear brighter as a result.
By the way, F/1.4 lets in four times more light
than F/2.8 for given focal length.
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