70D Liveview AF hunting

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DannoHung
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70D Liveview AF hunting
2 months ago

I got a 70D recently and have been having a weird problem with the liveview AF. I'm using a Sigma 30mm 1.4 and when the liveview mode is on, the AF seems to work absolutely perfectly, and switches around seamlessly in single point mode depending on where I tap.

However, when I depress the shutter to actually take a picture, the focus hunts and invariably front or back focuses. The situation is no better when I switch the "Tap to shoot" mode on in Live View. If I let the camera run in live view mode, zoom in for a 10x crop (which is only ever marginally worse than I can get with hand focus) and then press the shutter, the focus jumps hellaciously back and forth and settles in a bad focus.

I am at a loss. The only thing I can do is let the camera settle on a focus point in live view, disable autofocus, and take the shot, but this is rather annoying, of course. I've been through a bunch of camera settings and looked for a lot of info and I can't find anyone describing a similar problem with the 70D. It's especially bothersome since the camera seems to be finding a focus fine, so it's not obviously a problem with the AF being out of alignment.

BTW, AF through the viewfinder on the same lens is fundamentally fine. I did have to use AF-MA (+8 on this lens), but after doing so, I'm getting rather nice, reasonably consistent focus. Not quite as in focus as the live view is before I press the shutter, but of course the viewfinder doesn't use the Dual Pixel AF system.

Any ideas? Do I just have a lemon? It doesn't seem to be happening on my other lenses of various makes and models. Although none are quite as wide open as my Sigma. Is there anything I could do to isolate the problem further?

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Canon EOS 70D
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