David Busch’s Canon EOS 7D Guide to Digital SLR Photography Quotes

“WATCH THAT QCD POSITION! While I was writing this book, I hosted a lighting seminar for neophyte photographers using cameras of all breeds, and out of 30 photographers in two sessions, no fewer than four Canon shooters were having trouble setting the aperture when using the Manual exposure mode I was having them use while working with studio flash units. (Each of them rarely used Manual.) All four had accidentally set the QCD switch to Lock (if they were 7D owners) or to the On (only) position (if they were 50D or 40D users), disabling the Quick Control Dial. I expect that this happens more frequently than I suspected, so I’m calling it to your attention once more in these two sidebars.”
― David D. Busch, David Busch's Canon EOS 7D Guide to Digital Photography, 1st ed