How Live View autofocus works: what your camera’s manual doesn’t tell you

Contrast AF vs Phase detection AF: key differences explained

Contrast AF vs Phase detection AF: key differences explained

Contrast AF vs Phase detection AF: key differences explained

Click on the infographic to see the larger version.

Live View contrast AF works on a different principle to regular ‘phase-detection’ autofocus sensors. Here are the key differences…

Phase-detection autofocus
Phase detection autofocus sensors compare two offset images of the subject. If they don’t line up, the subject is not yet in focus. This system provides much more information than contrast autofocus.

The distance between the images tells the camera how much it needs to refocus by, while the relative positions of the two images tell it whether it needs to focus nearer or further away.

The camera knows the correct focus position even before it starts refocusing.

Contrast autofocus
Contrast autofocus doesn’t offer this head start. The camera can only find out which way to focus and how far by trial and error.

It might refocus the wrong way before going the right way, and it will have to go past the peak focus point to find out where it actually is.

This happens comparatively quickly, but phase-detection AF nevertheless still offers a speed advantage.

Live View autofocus is accurate, but it requires patience and is better suited to fairly static subjects.

PAGE 1: How Live View autofocus works
PAGE 2: Contrast AF vs Phase detection AF – key differences explained

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