1. When your camera is on a tripod. Some folks still use autofocus when their camera is on a tripod, and for the life of me I can’t figure out why. A quick flip of the switch turns AF off, and then you can compose correctly before manually adjusting the focus—which is likely to stay consistent since your camera’s on a tripod.
2. When your subject isn’t moving. In much the same way as when your camera’s on a tripod, when your subject is stationary it’s often easier to focus manually. Rather than refocus after every image—a waste of time if the subject hasn’t moved—focusing manually allows you to set it and forget it.
4. When your subject is at infinity or beyond. Ever notice that your camera lens focuses to a certain distance beyond which everything remains sharp? This "infinity" distance may be only 30 feet away, but in optical terms it’s infinity nonetheless. In that scenario, a person standing 31 feet from the camera would be as sharp as the moon in that same shot when the lens is focused at infinity. So if your subject is beyond your lens’ infinity distance why bother struggling with autofocus? Just focus manually at infinity and get on with getting the shot.
5. When the light is low and your camera’s struggling. A camera’s autofocus capability requires a subject with enough illumination and contrast to function. That means in low light the camera can’t see so well and you may find your autofocus hunting for traction. In these situations more than any other, I make sure my camera is set to manual focus so that I can focus by eye and use the in-camera focusing aid to confirm it’s sharp.