"William Robb" <[EMAIL PROTECTED]> quotes:
> "The f number of the lens equals the focal length divided by the
> diameter of the entrance pupil of the aperture".
...and it's important to note, that the "entrance pupil" is _not_ the
physical front element of the lens (although with long focal lengths
(and in the extreme case of a single element lens) it will be equal to
it). It is, rather, the apparent optical diameter of the lens, which
you can observe by holding the lens, /sans camera/, aperture blades
wide open, with a light surface behind it, and looking at it from the
front side, from a distance. Notice that in the case of a wide angle
lens, the apparent optical diameter is smaller than the front element.
Basically, a real, composite lens is the optical equivalent (modulo
imperfections) of an imagined single element lens of the specified
focal length, and of a diameter equal to the focal length divided by
the specified max aperture number. This non-existent lens is what you
"see" in the above experiment.
-tih
--
Puritanism -- the haunting fear that someone, somewhere, may be happy.
-
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