Which Canon bodies can autofocus in the middle of a burst?

I think you are referring to AI-Servo mode, which I know the rebel XS (the current entry level rebel) has. Not sure about the older rebels but I would guess they do too.

They all have it to some degree. As you might expect, the difference is in how much control you get.

On a Rebel/EOSxxx you'll probably find that you can only access it in certain modes. From memory that would be the "Sports" mode. On my 300 (original Digital Rebel) it wasn't available in any of the "creative" modes.

With the 50D, and presumably above, you can select the focus method in almost all of the modes.

Chris7 said:

I would like to lock focus on a face or an eye, and have the camera keep re-focusing in the middle of a burst as the subject moves.

Can any of the current Rebel series continue re-focusing in the middle of a burst, or is this feature reserved for the 50D or 7D (or higher)?

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Yes, all dslrs can do that, including entry-level dslrs from Canon. To my knowledge Canon calls this AF mode `AI Servo' while Nikon calls it AF-C (= continuous AF, a more intuitive name, but anywho).

One word of caution: even though you have this ability built into your camera, to master it is quite difficult. Since I don't use Canons, I don't know all the different settings (I'm sure they exist in one form or another), but on my Nikon, you have 3 x 3 different AF modes, on the bigger models it's at least 4 x 3: to know which mode you want to use requires some practice.

On the other hand, with kit lenses, you will have less focus issues as the depth of field is comparatively large. It's much more important with more expensive lenses, e. g. primes with large apertures or tele lenses. With my bazooka zoom, I notice if the focus point is just a few mm off (at a distance of, say, 2-3 m).

Chris7 said:

BTW, I heard that Nikon was somehow better at predicting movement and maintaing focus on a moving subject, but my friends with whom I share lenses shoot Canon, so that's what I'm using. Any truth to Nikon having superior focusing ability on any of their crop sensor cameras?

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There is anecdotal evidence that Nikon tunes its autofocus systems to value accuracy over speed. I've seen this in several head-to-head tests, but even if there is a difference, I don't think it matters as much with today's cameras.

Currently, I can think of only one Canon camera that is at a significant disadvantage when it comes to the AF system and that's the 5D Mark II: it essentially retains the fairly ancient AF system of the 5D and even the Canon 40D/50D feature a more beefy AF system than the 5D Mark II! Since you're not interested in this camera, I don't think it's particularly relevant.

Other than that, I think neither company has a significant edge when it comes to AF systems. In any case, not significant enough to switch from C to N or vice versa.

AI Servo is predictive AF. as long as the AF button is held down, it will be tracking the subject, and when the mirror flips up for an exposure it guesses where the subject will be in the next instant (when the exposure is made). it re-orients itself when the mirror comes back down.

on the 7D and 1-series, you can choose if you want drive speed or tracking priority, and how long before the AF refocuses when an object obstructs the view to the original subject.

toxic said:

AI Servo is predictive AF. as long as the AF button is held down, it will be tracking the subject, and when the mirror flips up for an exposure it guesses where the subject will be in the next instant (when the exposure is made). it re-orients itself when the mirror comes back down.

on the 7D and 1-series, you can choose if you want drive speed or tracking priority, and how long before the AF refocuses when an object obstructs the view to the original subject.

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AI-Servo isn't predictive. It's a continuous autofocus. As long as you continue pressing the shutter release, the camera keeps the lens trying to focus on the moving subject.

Predictive would be using the camera set on single-shot and off AI-Servo, and then trying to have the camera focusing on a moving subject. In this case you are trying to anticipate where the subject will be when you press the shutter button.

Re-Edit: I was reading about AI-Servo at another forum, and you may be correct about it being "predictive focusing." I apologize.
http://photonotes.org/other/ai-servo.html