In article <5ls7jq$80s_001@leeds.ac.uk>, CEN5JDL@leeds.ac.uk says...

>How do you get optimum image from a lens? I heard that you'd
>set the aperture 2 stops down? Whats that mean? I currently
>have a Canon MkII 50mm f1.8.

I keep waiting for that bad ol' "2-stop-down" chesnut to
go away, but it lingers (though it is only correct for 35mm
format lenses if the lenses happen to be f4 wide open...;-).
Most good lenses for 35mm peak over most of the frame around
f8 - poor lenses (and some zooms and superwides) may require
f16 (or smaller!) to optimize the edges/corners. The above
refers to lens image quality, and not to overall photo image
quality (f4 or f16 [etc...] may be more appropriate for
optimizing the photo image for DOF reasons. BTW, most 50mm
lenses are fine from f2.8 to f16, with subtle improvement
visible in the area around f5.6-8.
"Hope This Helps", "Bob Neuman", David Ruether
http://www.fcinet.com/ruether