So last Saturday I shot a drift event at our local track.

Had a real bitch of a time with my autofocus on my Rebel XSi. Didn't matter what lense I was using, tried my L, nifty 50, etc.
I made sure i was in ai servo and panning, but it miss focused a lot! I mean like a WHOLE unacceptable amount of..umm... lot.
I cleaned the sensor contacts, no change...

Any advice?

What really sucks is this is my newer body, my old XT nailed the focus every time with any lense and is well on its way to 100,000 frames.

you using all points or are you picking?

I'd go find an interstate where you can park and watch and practice though with both bodies?

I had selected my point for the AF.

The body is not new to me and has never "acted up" prior to saturday.

The only thing I can think of is maybe the closer then normal proximity to my rapidly moving subjects threw it off?

yeah, weird. How close we talking?

you tried all your lenses, so that basically rules out an AF limiter switch...

anywhere from 3 feet to 20 feet+. Depended on the corner and how ballsy I felt.

I used all but 1 of my lenses (left my Tamron 28-75 f2.8 at home).
So that means I used:
10-22mm-Sigma f4-5.6
50mm f1.8-Canon
70-200mm f4 L-Canon
500mm f6.3 Pro-Optic- Manual focus so it doesn't count.

http://www.flickr.com/photos/shawnma...th/4946402343/

I just went through ~20 shots at flickr's normal size and didn't see a SINGLE shot I'd say the camera missed focus on... Pick out some you think are ESPECIALLY bad and post them here for us, but I think you're having an issue with panning and preventing camera shake while doing so, not actual focus... Panning is a skill that takes lots of practice and where IS can REALLLLLY help. But even if the AF is NAILED, the whole camera is rarely in focus unless the whole car maintains the same distance for the length of the pan...

lol
Thats 'cause I deleted the bad ones...Im not about to share a crap shot!

I understand the car will not be entirely in focus ( I do a LOT of motorsports work).
The troublesome shots are where I could see the AF failing through the view finder, call it 'hunting' if you will...like they do in low light w/o an af assist.
Panning is tough, especially down at 1/80 like I like to do, but I've never in the last 10 years had to 'bin this many images due to failed af.

I guess live and learn, save up for better/faster af bodies and glass.

yeah, it's tough to say really then.

I appreciate your help!

Shawn

considered driving out with Angerman for that event but it was too short notice to get off of work the extra few days

Focus seems fine to me like ben said, if you say it was hunting I'll take your word for it and suggest that tire smoke could have been the issue.

The blurry photos I see on flickr appear as though you weren't keeping the af point on the same part of the car for the entire exposure; while panning in AI servo with the shutter release depressed half-way if the point your focusing on keeps changing the distance will be readjusting constantly so that could also be the "hunting" you noticed.

I used to shoot at slower shutter speeds a lot but doing so just doesn't lend itself well to drifting in bright sunlight; the cars move too awkwardly in that length of time and without a filter you're at f/16+ the whole time. Now I tend to stay at 1/160-1/250 range using a CPL and get mostly the same blur with sharper photos at a higher keep rate, which means less shooting is necessary overall and less time is spent in post.