The 40D has three AF modes: One Shot AF (also known as Single Autofocus), AI Servo (Continuous Autofocus), and AI Focus AF (which switches between the two as appropriate). I’ll explain all of these in more detail later in this section.
With manual focus activated by sliding the switch on the lens, your 40D lets you set the focus yourself. There are some advantages and disadvantages to this approach. While your batteries will last longer in manual focus mode, it will take you longer to focus the camera for each photo, a process that can be difficult. Modern digital cameras, even dSLRs, depend so much on autofocus that the viewfinders of models that have less than full-frame-sized sensors are no longer designed for optimum ...