1.4 extender MkIII and AF accuracy

I'm using a 1.4x III with a 70-200 F4L IS on a 60D body, and I'm finding that in general AF accuracy seems to suffer. The lens itself generally hits the target OK, but with the extender things seem a bit more erratic --- some times in front, some times behind, and rarely on-target first go unless the subject is particularly simple. How does this square with others' experience?

MFA is the answer.

rndman wrote in post #18063178
MFA is the answer.

Unfortunately this doesn't work on a 60D, or when there's no systematic front/back bias.

I have used it briefly on the 70-200 f4 IS but did not notice AF problems. I have used it more on 300 f4 IS and extensively on 100-400 II and AF slows but is accurate. More easily confused I would say, ie on a difficult subject but that's harder to quantify.

ettlz wrote in post #18063228
Unfortunately this doesn't work on a 60D, or when there's no systematic front/back bias.

That was the reason I got rid of the 60D - lack of the MFA.

However, if you cannot achieve a consistent focus, it can be an issue with either the lens or the camera. Do your other lenses work well on the 60D? Does this 70-200 work fine on your 7D? If yes, it is the 60D issue. In fact, my 60D, when I had it, went to Canon twice and the AF circuit was replaced.

"The lens itself generally hits the target OK"

Any Canon 70-200 should always AF very accurately on any Canon DSLR unless poor light or difficult subject.
There is no easy way to define poor light or difficult subject but good light and easy subjects are easy to recognise.
How is the bare lens in good light and easy subject?

Choderboy wrote in post #18063467
"The lens itself generally hits the target OK"

Any Canon 70-200 should always AF very accurately on any Canon DSLR unless poor light or difficult subject.
There is no easy way to define poor light or difficult subject but good light and easy subjects are easy to recognise.
How is the bare lens in good light and easy subject?

It's a bit weird actually. With a simple target, on either body, when focusing from near to the target, it's dead-on about 9 times out of 10. But focusing from further away back to the target, about 1 in 4 shots it overshoots and ends up slightly front-focused...

The AF in my naked lens 70-200/4 non IS was great. My 1.4III is great but with prime 400/2.8 L IS. The combo 1.4III+70-200/4 was good, but the hit rate was significantly lower.

It sounds like the lens is not quite right. At least you have a good description of problem if you do want to try getting it fixed.

Don't forget that with any extender you have the probability of reduced speed and reduced accuracy of auto focus. Some lenses work better than others with extenders, but I'd put that down to them being exceedingly excellent in the first place so a little degradation is still going to result in really, really good performance

Choderboy wrote in post #18063686
It sounds like the lens is not quite right. At least you have a good description of problem if you do want to try getting it fixed.

Yes, trouble is it's one of those annoying intermittent things that mysteriously doesn't show up when sent in for service.

I have never had any problems using extenders other than the expected slow down of the autofocus.

Sounds like you have an issue with something to me.