I've heard negative things regarding this combination. I've also read where a couple people had decent results. Someone on another forum even claimed it would still autofocus.I had to find out for myself. It arrived in the mail today. Well, it doesn't autofocus. However, I was prepared to manual focus. I tried it a couple times last week and it wasn't that bad. It's certainly going to take practice. Here are a couple tests shots from this evening. Until I get more experienced, I'll definitely have less keepers. I was quite pleased the way these 2 turned out. On each of them, I cropped out about 65%. Would this be considered a 2/3 crop or 1/3 crop? This has always confused me. Anyway, I retained about 1/3.

The oriole was shot handheld at 560mm at 1/125 sec.

Wow Mike! That's very good! Hand held!? Yikes. You've got very steady hands! Looks great!

Here's what I copied from another site post about the auto focusing on the extenders...

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And here's more information...

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Hope this helps...

Naughty, naughty....

My bad....bad habit, I keep forgetting it. Sorry!

I picked up a Kenko 1.4 TC for my Nikon because I had heard that the autofocus would work. And the autofocus works but it is quite a bit slower and it only focuses well in bright light. Nice work X. I think you'll like the combo. I'm waiting for the Sigma 150-500 OS to finally hit the shores so I can see the reviews and decide if that is the route that I want to go.

It was the Kenko that I purchased. After rereading the post on another forum, I see now that it was a different model Kenko that the user stated still offered AF with this lens. However, he also said the AF was slow and did a lot of searching. In that case, I think I'd be better off learning to manual focus. It's just another aspect to try and master. The TC is not something I'll probably use a lot, but at least it's there for when I need it.

Mike, these are nice quality with the TC.

The best TCs will only autofocus on the 100-400 if you have a "1" series Canon body, because that series will autofocus a lens/TC combo with a max aperture of f8. All other Canon bodies will only autofocus lens/TC combos with max apertures of f5.6.

It's a matter of what the TC allows the lens to report to the camera. All top TCs fully report aperture to the camera, so the topline Kenko and the Canon TCs tell it like it is. Some lesser quality Kenko and Sigma and Tamron TCs don't report aperture to the camera, so the non "1" series bodies will autofocus the combo. But since that zoom can't afford the loss of IQ that comes with a TC (except in very good light, where I've gotten some nicely sharp images), putting a lower-quality TC on it just to maintain autofocus will compromise IQ too much, IMO.

The topline TCs can be spoofed by putting a small piece of electrical tape over three of the electrical contacts (pins) on the TC, which keeps the TC from reporting aperture to the camera, but as said it will force the autofocus to hunt quite a bit, and my reading indicates that taping the pins causes the lens' focus motor to wear out faster.

X,you may or may not know this,but when manual focusing with the T/C,you should be able to hold your shutter button half way down and still get a focus confimation light and "beep" when it locks onto a subject.

I've never gotten the beep or the light on manual focus with that combo and a non-"1" body.

Huh, that's weird,mine does. ????

I believe it depends on if you have the camera set to one shot or high speed mode. I leave mine in high speed so I never get the confirmation.

That is odd. I'll have to try some more in case I'm doing something wrong.

The lens needs to be set to manual focus on mine to get the confirmation light/beep with the t/c.

Thanks, MM. I hadn't been doing that. I'll give it a try.

Well, I tried everything and can't get the confirmation light and beep. I reset my camera to one shot and the lens to manual focus, and still nothing. It may depend on the brand TC used. I have the Kenko.