Autofocus in Live View

I was just playing around the other day, and put the 2X on the 100-400 II (with the 7D2). I sometimes use the 1.4X on it and have been pleased with the results, and wondered what sort of garbage I'd get with the 2X. Of course it won't AF, so I went to Live View to zoom in and try to focus, and I was surprised to see that it will AF in that mode. It's slow and probably not pinpoint accurate, but it never hurts to have these things in the mental database. I may try to find some use for it -- might garner an image for artistic processing. (But then, why not just crop a 1.4X image....)

Just thought I'd toss out the observation. Live View is a tool most of us have and probably rarely use. I usually only think of it for focusing the IR-converted 5D2.

Actually it should be very accurate since live view uses contrast detect to AF.

I think I should have said precise instead of accurate. I'm wondering how pinpoint the focus can be.

A converter only magnifies as it has no 'workings' so to speak, so agree with Don, it should be accurate.

I have a Canon 7D and use a Sigma 500 f4.5 EX lens. I have found the live view AF to be inaccurate with my bare lens. I do not know if it is a fault with my lens or camera body. My lens will not AF with converters even with a pro body attached. If and when I attach a 2X tele-converter it will be for a static bird. I will auto focus on the bird, switch to manual then attach the converter then magnify the image on screen (which can be very difficult with the vibrations induced by touching the focus ring) and then make any fine adjustments necessary. At such extreme focal lengths I mostly use a cable release and bring my tripod down as low as possible to maximise stability. The results can be a bit hit and miss and I will only go through the ordeal if I think a real benefit can be achieved. The results I have had cannot be too bad since I have had images published using the combination.

I do not know how a Canon 100-400 II will respond if you have everything going for you i.e., good light and a cooperative bird, but I imagine you may be pleasantly surprised. Of course there will be a slight drop in IQ, in which case, the subject matter needs to rise above the mundane. I would certainly experiment and practice with your combination.

I had that lens years ago, and the matching Sigma TCs. It was quite sharp even with the 2X, but focus was manual. I used it back in the days of the Canon 20D and then the original 5D. Things have improved a lot since!

I'll probably play a little more with the new 100-400 and the 2X TC, but I think I would favor cropping with the 1.4X. If I had the chance, I'd definitely use the 600 II + 1.4X and even 2X.

You're quite right about the difficulty of keeping shake down to focus manually at long focal lengths. And once focus is set, definitely a remote release in Live View with time for things to settle down. And no breeze, please! In the rare cases it would be practical, using a laptop and Helicon Remote lets me drive the focus motor by as little as a single step on a magnified view. That's how I focus for astrophotography.