The Quick Control Screen on EOS DSLRs makes changing settings via the rear LCD quick and easy - just press the Q button to access it. However, if you're trying to use the Quick Control Screen and it doesn't seem to be working, it's most likely because you have the Custom Function for the AF Point Selection method set. If C.Fn III-3 is set to Option 1 for Multi-controller direct, the Quick Control Screen won't be accessible disable the Custom Function and you can use the Quick Control Screen again. The new Canon 5DS also offers a Custom Quick Control Screen (see screenshots below) you can edit and customize the contents, size, and placement of the information fields so that only the functions you really need to see are shown.

Custom quick control screen

Q mode


The Quick Control Dial on the back of many recent Canon EOS DSLR cameras have a number of functions, but essentially, once a function (such as white balance, AF, drive modes, or flash exposure compensation settings), or any menu tab that has been selected the dial enables you to quickly scroll through the options to choose the settings you wish to adjust.