The focusing mechanism in lenses incorporates a rotary encoder which provides the distance information to the body for use with the ETTL system. It is not what could be called a precision measuring tool. If you want to see what the resolution is, try mounting your flash (ETTL mode) in the horizontal position. As you focus at various distance observe the distance information displayed on your flash. The distance information is not displayed when the flash is tilted up for bouncing as the camera has no means of knowing the distance the light must travel.