Do grips help with autofocus?

There is no more battery power available when you use a grip on today's cameras. The camera manages the grip by using the batteries one at a time, alternating between the two. So from the point of view of powering the camera, it's the same with grip or without. But it will run more hours, since there are two batteries on line at the same time, so to speak.

Never had a grip before. Actually this grip came free with my 5D3. My original intention was to sell it - to effectively reduce the cost of the camera.
Being left handed it is invaluable in helping one hold a heavy camera and a reasonably weighty lens.

It also helps with AF, in that in Portrait mode, the "M-Fn" button is in the right place to quickly change from Single Point AF to Point AF.

But this does not address the OP's point.
Battery power should be the same, not sure in the case where a battery is on its very last legs; would the second battery help?
But in general use agree totally with all the above posts.

But overall for me its a superb accessory - now regard it as essential.
At low shutter speeds, the better hold must help me to hold the camera a bit more steady.

John_N wrote in post #16309613
Thanks I hadn't thought of that, plus the keep fit part where a Brummy trucker screeches to a halt and hunts me through three counties will be exhilarating

LOL - if he catches you you will be badly beaten with a Yorkie bar (which is a Chocolate bar, for those non-Brit forumites).

I have grips on both my cameras. Does it help speed up AF or make it more accurate? No. I have found that it balances longer lens better, which in turn make locating and tracking easier. Just my opinion, so take it for what it is worth.

It sounds to me like a lens AF speed issue too. The 5D3 has fantastic AF capabilities, but if the lens is physically too slow moving to keep up with fast subjects there is nothing the camera can do to help it. My 100-400L is also slow to focus, when compared to my 300 f/2.8L, and can have the same issues. That would explain it locking on at a distance (particularly if you start the AF before the bird is released) then loses it because the lens is not able to adjust quickly enough.

A couple of questions:

Were you taking single shots or in burst mode? If in burst mode, how is your camera set for "AI servo 2nd image priority"?

If that is set to speed (or even equal) then the camera may be shooting too quickly. That should be set to "focus", which should slow down the frame rate to allow the lens more time to adjust between each shot, and the camera should only shoot when focus is achieved. You could try and track the bird, without shooting, until it gets to where you want it, then take a single shot. that should allow the lens time to focus and your one shot should be sharp (assuming a fast enough SS of course). Alternatively, do it the old fashioned way and set the lens to MF, prefocus at a set distance (ideally with a recognisable marker in the scene) and shoot as the bird flies across that point.

Where were you standing relative to the bird?

If you are standing close to the intended landing point, to get a head on shot as it flies, you are going to have some very serious changes in distance to deal with between each shot and the lens will probably struggle with that. If you stand well off to one side and pan with the bird the distance changes much more gradually as the bird is going more left to right (or vice versa) rather than straight towards you, that makes it much easier for the lens to keep up.

I am assuming here that it IS simply missed AF and not softness due to too slow a shutter speed. A fast moving bird coming towards you needs a FAST shutter speed.

As always, posting a couple of images that show the problem makes it much easier for others to see where the problem may lie and give the best advice. If you post one or two it may help narrow down the issue.

Thats a fair set of questions - just checked the camera and here go the answers:

Set to burst H - 2nd set to equal priority, so I'll have a good think about those - MF isn't really my strong point so would almost certainly fluff that.

I was stood roughly five feet to the right (otherwise a bloody dead hare would have slapped me in the face!), so roughly head on.

Yep shutter speed was 1000+

Sadly when the worst examples happened it simply didn't fire so I have no image to show, but were I to it would have been a massive blur!

There were a couple of other times always when the bird was coming toward me - panning is usually pretty easy and I almost always get those as the AF has to do so few adjustments.