I recently bought a book about exposure in photographs. In the book, which contains some stunning and well exposed shots, the photographer said that he uses "M" mode all the time. He sets the aperture manually, then adjusts the shutter speed until the camera tells him the exposure is right. I totally understand how that works and the relationship between aperture and exposure. However, am i missing something by not understanding the difference between his method and Av? When using Av doesn't the camera adjust the shutter speed automatically to suit the manually chosen aperture? If so, then it's quicker than using "M" mode. My understanding is the same with Tv mode except in this mode, the photographer manually sets the exposure and the camera adjusts the aperture automatically. Can anyone explain to me why he, or any photographer, would use "M" mode when not looking to alter the exposure of an image?