The first time I tried changing the aperture on a modern Canon SLR (a 20D) I was mystified. I tried turning all the knobs while on manual mode, and nothing would do it. I eventually broke down, got the manual, and learned the on/off switch in the back had to be moved past the “ON” position onto a line which pointed to the scroll wheel. Since that day, this behaviour confused me to no end. Why should the on/off switch not be turned to on when changing the aperture?

Looking at pictures of the new Canon EOS 7D, it finally dawned on me. The on/off switch has been moved, and where the “ON” switch once was is now labelled “lock”. The scroll wheel is too easy to accidentally move, so the old “ON” position functioned as a lock to prevent accidental changes. I probably should have realized this earlier, but I don’t use Canons often enough to have done so.

I still really don’t like this interface. For one thing it’s a needless extra switch, where as on Nikon bodies we just change another wheel with our forefinger, without chance for accidental input. The real issue, however, is for people like me who use their left eye in the viewfinder. This means the scroll wheel, which is on the right side of the camera body, gets jammed up against our faces. It’s just not a comfortable position from which to change the aperture.
Tiny link: http://taii.cc/2R