Autofocus and how it works: the modes

A regular reader asks (and I take questions, you know that, right)?:

Can you possibly do a short blog entry on the focusing modes offered by Canon? You know the ONE SHOT – AI SERVO – AI FOCUS buttons?

Certainly!

These “focus modes” are not about where the camera focuses (the focus point selection is about that: select one point for accurate control). Instead, these modes are about how the camera focuses. There are two main modes and an in-between hybrid mode:

  1. One shot (Nikon calls this “AF-S”)
  2. AI Servo (Nikon calls this “AF-C”)
  3. AI Focus, where the camera tries to guess which of the above two you want.

“One Shot”/AF-S means: when you press the button, the selected focus point tries to achieve focus. Once it does, a few things happen, all at the same time:

  1. The camera beeps
  2. A green dot lights up steadily
  3. The focus locks, and until you remove your finger, it stays locked at the selected focus distance.

This One Shot/AF-S mode works well in most cases. But what if the subject you are shooting is moving rapidly towards you, or away from you? Then every picture will be blurred, because by the time you press, the object is no longer where you locked the focus.

So for those situations you have AI Servo/AF-C (AI Servo means “Artificial Intelligence Servo motor control”, while AF-C means “Autofocus -continuous”). The focus never locks, and the camera keeps buzzing away as the focus motor keeps turning and adjusting. Better, it even predicts where the object will be when the shutter opens (that is the “artificial intelligence” part). So you can try to use this mode when shooting “sports, birds, and kids”.  Not every shot will work out, but at least some will.

Finally, the hybrid mode (AI Focus) tries to guess which one of the two modes above you really want, and then switches to that. I am not a great fan, and expensive cameras like my 1-series models do not have this mode. I think you should probably decide whether you are shooting mainly stationary subjects (then choose One Shot), or moving objects (then select AI Servo).

1 thought on “Autofocus and how it works: the modes

  1. Hi again Michael,

    If, for example, I am shooting in “One Shot” mode and If i lock focus on a certain subject and then recompose the shot. I don’t release the button but simply recompose the frame. Will it still remember that it has focus locked on the original focus? Does that even make sense?

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