I can't say for a Canon but on a Nikon, when using the automatic lens that has no aperture ring, if I want to use manual settings, here's how I must go about it. I turn the mode dial to A for aperture priority, spin the control wheel until the lens is set at the desired f stop, then switch to M for manual. In that poistion, the lens will remain at whatever f stop I set it at and the control wheel then controls the shutter speed. If the image is too bright, I spin the wheel for a faster shutter speed and vice versa. With a non-automatic, old style manual lens that has an aperture ring, I simply set the aperature at the desired setting. In the M manual mode, the control wheel still controls the shutter speed.