In article <4hiuf7$t71@decaxp.harvard.edu>, kkobayas@scws45.harvard.edu says...

>The F number (focal ratio) is focal length divided by aperture. Here
>"aperture" is diameter of the front element. (I always get frustrated
>that people use the word "aperture" to mean "F number")

Ummmm, If the aperture is the diameter of the front element, there are
a lot of much faster than rated SLR wide-angles, and somewhat faster than rated normal lenses around! ;-)
Hope This Helps