Lens will autofocus on XSi but not 40D

[khuckaby]

I have a Canon EF 70-200 f/4L USM that works great on my XSi. I have been looking to sell to I can buy different toys. A potential buyer put it on his 40D and could not get the lens to AF unless it was close to being in focus already. He had other lenses with him that seemed to AF fine.

Some differences with how I shoot and how he tested. I am always outdoors shooting sports. He was inside a darker Starbucks shooting across the room 15 to 20 feet.

Is it him, the lens going bad, his 40D??? Any help is appreciated.

[LeeOtsubo]

Did you test it with your XSi at the same time. IOW, it worked w/ your XSi but failed on the 40D moments later?

Did his target have enough contrast for the phase detection AF system to work? Did you try stepping outside into sunlight to test the lens? Was the lens set to AF? Did he try cleaning the contacts? Had he set a different AF point and forgotten? Was AF set for All Points Active and grabbed a different point then he anticipated? When AF fails, 99% of the time, it's operator error. In my workshops, I spend a lot of time teaching people how to correctly AF.

I've had the same lens for nearly 10 years (it was introduced in 1999) and it's a workhorse. The only one's I've seen malfunction were abused.

[khuckaby]

Did you test it with your XSi at the same time. IOW, it worked w/ your XSi but failed on the 40D moments later? No, I didn't have my camera with me. I thought this was going to be quick and simple so I left my equipment at home.


Did his target have enough contrast for the phase detection AF system to work? I was worried about this, but didn't give it too much extra thought.

Did you try stepping outside into sunlight to test the lens? Was the lens set to AF? It was 9PM and dark outside, so that wasn't an option.

Did he try cleaning the contacts? We wiped them, but not a good cleaning.

Had he set a different AF point and forgotten? Was AF set for All Points Active and grabbed a different point then he anticipated?

I don't know the answer to these. I will do some more testing and have a friend test it with their camera.

Thanks

[LeeOtsubo]

Just for grins, I mounted a 70-200/4L on a 40D and tried focusing on various low contrast targets. With the focus range limit switch set to 3m, the lens at 200mm and the target at ~8-10ft, AF was sketchy. I know you said 15-20ft but when I set All AF Points Active, it was focusing on all sorts other higher contrast targets. As I said, 99% of all focus problems are operator error. Did this guy know what he was doing or was he a newbie? Just because he's had the 40D for a while doesn't mean he knows how to use it.

[khuckaby]

After further testing I am convinced that it was an OE (operator error) and not an AF problem.

Thanks for the help