Canon EOS 5D Mark III | First Thoughts

[neuroanatomist]

Interestingly, fast glass means a more accurate AF lock, but not necessarily a faster one. An f/5.6-sensitive AF point will actually achieve a lock faster than an f/2.8-sensitive point. With an f/2.8 or faster lens and a dual-sensitivity AF point (either an f/2.8 line + f/5.6 line cross type, or a dual cross type), focusing is a 2-step process - an initial focus move from an f/5.6 sensor line, then a refinement by an f/2.8 line. In AI Servo mode, most of the work is done by the faster (but less accurate) f/5.6 sensor lines.

[shizam1]

With an f/2.8 or faster lens and a dual-sensitivity AF point (either an f/2.8 line + f/5.6 line cross type, or a dual cross type), focusing is a 2-step process - an initial focus move from an f/5.6 sensor line, then a refinement by an f/2.8 line.

Well that is interesting, because in my low light tests, I saw the focus move a lot, then stop, then couldn't see any other changes for a noticeable chunk of time, followed by the AF confirmation. So maybe it was "refining" focus and that was taking up some time.

If that is the case, then in situations like that, I could take the picture after the initial movement, and take it again afterwards, just in case the scene I'm trying to capture dissapeared before the final AF adjustment.

[neuroanatomist]

Well that is interesting, because in my low light tests, I saw the focus move a lot, then stop, then couldn't see any other changes for a noticeable chunk of time, followed by the AF confirmation. So maybe it was "refining" focus and that was taking up some time.

If that is the case, then in situations like that, I could take the picture after the initial movement, and take it again afterwards, just in case the scene I'm trying to capture dissapeared before the final AF adjustment.

You could, at least it's worth a try! IIRC, there's a setting for focus priority vs. release priority, and you'd want the latter so you can release the shutter before focus is confirmed.