AF accuracy on the 5DIII is slightly better than the 1D IV. AF speed might be on par or a little less but even with BIF shots I didn't notice the difference at all. The benefit of using the 5DIII is that you will also get closed loop AF with Canon's newer lenses such as MK II models. Those lenses now report AF position back to the camera. However, even with the older lenses, the AF accuracy is improved.
For best AF accuracy for action, you also need to set the firmware to use only cross type af points. If you have it set to use ALL of them, it's possible for the AF to be less accurate...It's fine to use ALL of them if you are shooting static subjects.
Most people using the older bodies are accustomed to using the center point only. While that is still a very good option for most, the 5DIII offers a few other AF modes that are an improvement over using just the center point....and with most modest lenses, a large block of the center AF points are all cross type.
Lots of benefits to using the 5DIII for action....but a lot more options to tweak based on what you are shooting. It's a bit overwhelming. I've been using a 5DIII for about 1 year now shooting almost exclusively wildlife and I'm still finding tweaks that are improving the numbers of keepers in certain situations. Steep learning curve but glad I have it.
BTW, got rid of the 1D IV and 7D. Now waiting for the 7DII to come out as my 2nd body.
I apologize if this has been covered in the past but I did a quick search on the subject and could not find what I was looking for.
I'm hoping that someone out there has some real world experience and can answer my question.
How does the 5D MkIII AF compare to the 1D MkIV? I'm particularly interested in the accuracy and AF lock-on speed. My primary use is action photography; motocross and birds in flight.
The lenses I mostly use, in order of time on the camera are:
70-200 f/2.8L IS II
300 f/2.8L IS
I should mention that I do not intend to replace my 1D MkIV, which I have been using for 3 years. If I acquired a 5D MkIII, it would be a second body to use primarily at the motocross tracks. Most likely, I would carry the 300/2.8 mounted on the 1D MkIV and the 70-200/2.8 or 135/2 mounted on the 5D MkIII.
I am also not concerned with the burst rate as 6 fps would be adequate. I normally keep my 1D MKIV set at 7 fps for motocross and it's been fine.
Any help would be appreciated.