Where does the speed come from?

For a camera to capture fast action, where does the auto focus speed come from, a super duper lens, a super duper camera or both.
I have a 400D and a crappy 75-300 non IS which just cannot focus quickly enough to get in focus shots of whippets coming towards me at 15metres per second. Do I just need to get a better lens or would the 400D still struggle.

The 400D would still struggle. My 5D, which isn't a great sports camera but supposedly has good tracking AF, still wouldn't focus quickly enough.

To answer your other question, it is a function of both the body and the lens--USM lenses are generally faster focusing than non-USM lenses. Also, certain focal lengths coupled with wide apertures focus more slowly because you're moving more glass over a longer focal length--i.e., a 15mm FE is going to focus a lot more quickly than an 85 f/1.2L.

I'll second Markitos' comment - the camera and the lens both influence AF speed. My experience is that the 40D and 50D are slightly faster and more certain than the Rebel autofocus, because of a better sensor. Generally Canon USM lenses focus faster than third party lenses. Also, recognize that it helps to have a lens with a large maximum aperture, since this allows you to shoot with a higher shutter speed.

I have noticed from shooting all kinds of bodies that the speed which a lens focuses at is not all that dependent on the body itself. the old 18-55 was slow whether on a XT or on a 50D. THe 17-55 is amazingly fast on both bodies.

Where the body kicks in is when the subject is moving and the camera needs to track. Then there is quite a difference. The 50D tracks objects much more accuratly and quicker than say an XT. So, stationary objects = lens. Moving objects = body.

Thanks, you've told me what I already suspected, I need to buy a new camera and lens, goodie!

I'd say lenses are usually the factor in the beginning, once you have a few ring USM lenses then the body might be holding you back...

The focus is determined in the body, but it can only get focus when the lens' AF motor moves to the right place in response to its requests.

So what are you going to get, Whippeticious?

Both. My 40D did better with the 85 f/1.8 than it did with the 50 f/1.4. When I went to the 1D II, all my lenses became better focusers!

twoshadows wrote in post #8637265
So what are you going to get, Whippeticious?

I'd love to get the 1D mk3 but I really cant justify it. As it is, I struggle a bit justifying expensive gear when all I do after I've taken a photo is think Oh, that's a nice photo, and then.......... nothing. It's not like it can pay for itself, sadly, I dont have the talent, so it's just for fun.
I was thinking about a 50D and maybe, just maybe the 70-200 2.8 IS. Then I have to slap myself and say this is madness.

If you're not printing big and don't shoot above iso1600 much, why not the 1DmkIIn? It has a bit better AF than the 50D and a few more fps.

There is not going to be very much difference in focus speed between the consumer bodies. They are all depending on a single (albeit dedicated) RISC chip for the focusing calculations.

The 1-Series uses two chips dedicated to focusing, which makes them much faster.

twoshadows wrote in post #8637767
If you're not printing big and don't shoot above iso1600 much, why not the 1DmkIIn? It has a bit better AF than the 50D and a few more fps.

Or even a vanilla Mark II if you can deal without the screen

With the 7D im betting a few Mark II/Mark IINs are going to flood the market

Whippeticious wrote in post #8637548
I was thinking about a 50D and maybe, just maybe the 70-200 2.8 IS. Then I have to slap myself and say this is madness.

Excellent choice!
For my action shots I upgraded my 20D to a 50D and saw substantial improvement focusing & tracking action with a 70-200 2.8 IS. I find I'm shooting at a higher ISO (50D is much cleaner than the old 20D) for a faster shutter which is likely helping with the improvements. Since the 50D is better at high ISOs I can also afford to stop down to f4 and gain some DOF as a safety margin. It all comes into play.

If you're shooting the whippets at coursing meets shoot the other sighthounds and sell them to other owners to help pay for your habit, I mean hobby

Rubi Jane wrote in post #8638888
Excellent choice!
For my action shots I upgraded my 20D to a 50D and saw substantial improvement focusing & tracking action with a 70-200 2.8 IS. I find I'm shooting at a higher ISO (50D is much cleaner than the old 20D) for a faster shutter which is likely helping with the improvements. Since the 50D is better at high ISOs I can also afford to stop down to f4 and gain some DOF as a safety margin. It all comes into play.

If you're shooting the whippets at coursing meets shoot the other sighthounds and sell them to other owners to help pay for your habit, I mean hobby

Oh no, dont encourage me! I wont need much prodding to pick up the phone and order. argh! fighting to control myself.