60D Back Button Focus - AI Servo vs. One Shot - missed focus in AI Servo

Started Feb 7, 2011 | Discussions thread
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Schnapper
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60D Back Button Focus - AI Servo vs. One Shot - missed focus in AI Servo
Feb 7, 2011

Since upgrading from my Rebel XT (350D) to a 60D, I've switched to "Back Button" autofocus and leave the focus mode in AI Servo. I figured that it would be the best of both worlds - it can act like One Shot for focus and recompose by taking my thumb off of the button before recomposing, but then it's always ready and in "AI Servo" mode when I want to track the kids running around.

However, in low light I've noticed that sometimes it misses focus. It seems to think that it's focused correctly, as when I notice it through the viewfinder (or if I take the shot and look at the LCD) if I take my thumb off of the focus button and then push it again it still misses, and I think it's by the same amount. The other night, I was trying to get a shot and I couldn't get it to focus correctly after repeated pushes/releases of the button and attempts at focusing on a distant subject and then refocusing on the intended (closer - about 2 meters away) subject, so I switched to One Shot and tried again - it focused perfectly and I got the shot. While playing around, I also got out my 430EX II flash (mounted on hotshoe) and noticed that the IR AF assist only came on when in One Shot mode - not in AI Servo.

I've noticed the above with my 17-55/2.8, and I'm usually using the center point. I haven't done much testing with any of my other lenses or other focus points.

Has anyone else noted similar issues as described above in low light?

Is there a way to get the IR AF assist light on my flash to come on when the camera is in AI Servo mode? Do I have something set incorrectly, or is it normal for it to not come on in AI Servo AF mode? (I did check and have AF flash assist enabled... but maybe I only checked that setting when the camera was in One Shot mode? Is that setting specific to each AF mode?)

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