Suppose I am using a hand held meter to set exposure. I would ordinarily set the camera to Manual exposure mode, and set aperture and shutter speed according to my meter. Now, after doing so, I wish to have the freedom to alter either the shutter speed or the aperture while retaining the same exposure. I am using a (chipped) Nikkor AF lens, not one without "cpu" Easy, right? Just use the AE-AF Lock button (set to lock only exposure) and Bob's your uncle. Not so fast. In the wisdom of the Nikon programmers setting AE Lock does not lock exposure in Manual mode. If there is a logic to this behavior it escapes me.