vengence wrote in post #16719353
If it only happens with one lens (particularly a third party lens), I don't think you can blame the body for the problem. While canon isn't exactly great at sharing AF system with third parties, Sigma has added the USB dock to specifically upload firmware to the lenses for this reason. I'd suggest testing this with a couple of your other lenses to verify it's the one lens that has the problem.
Thanks for the suggestion except one problem, as I understand it, the 70d issues is isolated to lenses that are 2.8 or faster. I wish I had a barn full of fast Canon glass but for now, the 24-70 is my only fast Canon lens and the Sigma is the only other fast lens that I have. My other L's never seem to miss a beat.
I just went thru a sports season of shooting basketball games with the 70-200 f4L IS and the 24-70 f2.8L using primarily center point viewfinder and the hit rate for focus accuracy was near perfect for both so I was pretty sure the 24-70 would come out fine in these tests. Of course, almost all of the basketball shots were longer than 15'.
I agree that if the problem is only 3rd party lenses, this is not a Canon problem. My sigma, while having good sharpness at 1.4, great sharpness stopped down and great bokeh, has never been a well-oiled focusing machine. It sometimes fails to achieve focus even with well lit, very contrasty subjects. This is true on my 40d, 5d and 70d. The focus motor seems to skip and chatter sometimes as it steps around searching for focus.
Still, the 5' sigma failures will bug me for the 70d performance.