"John A. Lind" wrote:
[snip]
> What constitutes a "fast" lens is relative to the focal length. An f/2.8
> 180mm is considered "fast," but in a 50mm it would be considered slow by
> today's standards. Similarly, an f/2 28mm is "fast" but an f/2.8 "average"
> and an f/3.5 "slow."
>
> It's also relative to the film format. In medium format, very few standard
> lenses (75mm or 80mm) are faster than f/2.8. In large format view cameras,
> they're even slower yet.
FWIW, a little additional info: The "f stop" number is an expression of
the ratio of the lens's focal length to its aperture. Thusly a 50mm f/2
lens has a focal length of 50mm and an aperture of 25mm. As the focal
length increases (as in a telephoto lens), to maintain a constant "f
number" the lens diameter must be increased ... and the cost starts to
soar!
My 9.25" f/10 telescope (which I use for prime focus astrophotography with
my OM-1n) has a focal length of 235cm and an aperture of 23.5cm.
Have fun,
-Nick
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