Canon has extremely sophisticated focusing systems, but there are still people that can’t make them work. Most of the problems are due to the fact that they do not understand how Canon has designed them to fit all the various possibilities/combinations. I explained it on DPReview.com. Here, it is in a slightly different format.

One Shot

The AF assumes that the subject is static

  • If the subject is static, The AF will achieve focus immediately and that’s it. You can remove your finger from the back button, it won’t keep on focusing.
  • If the subject is moving, the photos will be out of focus.

AI Servo

The AF assumes that the subject is moving

  • If the subject is static, the AF immediately starts tracking the movements. However, the AF realizes that the subject is not moving and corrects itself to finally achieve focus. Which is why, often the first photo is out-of-focus and the others are in-focus.
  • If the subject is moving, the AF will immediately start tracking the movements.

AI Focus

The AF assumes that the subject is static

  • If the subject is static, The AF will achieve focus immediately and that’s it.
  • The subject is moving, the AF can’t achieve focus, the AF then switches to AI Servo. Which is why, often the first photo is out-of-focus and the others are in-focus.
  • If the subject is static, then starts moving, the focus was already achieved when the subject was static, the AF stops. The first photos will be in focus, the other photos when the subject is moving, will be out-of-focus.

What’s the difference between the One Shot and the AI Focus?

  • With the One Shot, the AF assumes that the subject is static and will not change mode.
  • With the AI Focus, the AF assumes that the subject is static but the AF will switch mode to AI Servo if it senses movements. But (and it’s a big but) the AF will NOT switch back to static if the movement stops.