I sold my cool old 35-year old, battered, manual only, non-stabilized, EF-converted, 400mm SSC f/4.5 FD lens to get a much more modern 100-400L IS lens, because I found that the lack of AF and IS was just too limiting for the types of pics I was taking.
I wanted a 100-400L, because I've always been so impressed by some very, very sharp pics from that lens, and the IS would be very useful for a shaky old dodger like myself!
But....despite the old 400mm SSC having some very visible scratches on an internal lens when they did the mount conversion from FD to EF, that lens was really incredibly sharp.
I expected the 100-400L to be even sharper, especially given the REALLY strong resolution that lensrentals.com tested the specific lens I was interested in.... one of the very few lenses with 22/22 lp/mm.
So, here's the difference, which I find pretty amazing.
The best of my "new" 100-400L shots of the moon. (Manual and AF focus ended up similar.)
_MG_9439-1-3 by Barry Timm, on Flickr
Not too bad, right? But not a lot of detail there.
Now, the battered old 35-year old 400mm SSC lens....
IMAGE LINK: http://www.flickr.com/photos/barrytimm/6327645194/
Taken with Canon 400MM Prime SSC f/4.5 FD converted to EF mount by Barry Timm, on Flickr
I sold the 400mm SSC for $195 incl shipping and insurance, and bought the 100-400MM for over $1100. HMMMMMM!!!!!!!
A little disappointed, to be honest, but the 100% improvement of usability and keepers from the 100-400L with its AF for anything moving and IS for non-tripod work, makes me "OK" with my purchase. If I'd never seen that old 400mm SSC, I'd probably have been happy as a pig in sh!t!