What does the focussing?

The body or the lens?

I know it's the lens that moves, what controls it and what works out whether point of the scene is in focus or not?

I've read that L lenses focus quicker - is it the lens though, or is it the case that it's (usually) bolted on to a decent body, and it's the body that speeds up the process of focussing?

Any thoughts on this would be much appreciated

The light from the lens passes through to the camera body where some of it gets reflected up to the viewfinder by the part-silvered mirror. The rest of the light goes straight through and is reflected down to the AF sensor. This uses a complicated procedure called phase-difference which determines how far out of focus the image is and how far the focussing mechanism of the lens needs to move to get the focus correct. The camera then tells the lens to move that distance and the process ends.

Once the lens has moved there is no checking to ensure the image is in focus, which some people find confusing.

The lens can affect focussing in two ways. First there's the type of motor, with the ultrasonic motor (USM) being the fastest. USM was more commonly found in the more expensive lenses but seems to be coming to be the standard. The second way a lens can affect focussing is that most cameras have at least one AF point (normally the centre point) that is more accurate with lenses that are f2.8 or faster.

There's an awful lot more on the whole topic of AF out there on the Interwebs - Google will find a lot of it.

AI Servo or AI Focus? Curtis N in post #2: "AI Focus is one of the most useless "features" of Canon cameras today."

What is meant by "presetting focus"

It's both. Think of your cameras and lenses as a high performance car. The speed and cornering is limited by the one thing that becomes the bottleneck. If you have a fast car with great suspension and tires, you're all set. But if a nearly identical car has lesser quality shocks or weaker springs, that one will be slow. Likewise if you replace the tires with smaller, cheaper ones, performance suffers.

Camera focus is the same deal. the time line of cameras, and the "professional level" of the body has built in a certain level of focus performance. You can, for example, lesson the quick / accurate focus of one of the high--end pro bodies by putting a so-so old lens, say one of the old 35-80's and end up with slower focus, softer shots, etc on the "best" body.

While putting expensive glass on a cheaper Rebel WILL certainly result in better shots than the same body with cheaper glass, that can only go so far, as well.

Yeah, it is both.

Camera tells the lens where to move and the lens moves there. The better the AF system in the camera, the faster and more accurately it can tell the lens where to move. The better the AF motor in the lens, the faster and smoother it can get there.

Thanks for explaining that

tkbslc wrote in post #9775060
Yeah, it is both.

Camera tells the lens where to move and the lens moves there. The better the AF system in the camera, the faster and more accurately it can tell the lens where to move. The better the AF motor in the lens, the faster and smoother it can get there.

Thanks for this; it also leads to something I posted about in another thread and have been trying to find confirmation on:

When people complain about their lens hunting for focus, isn't this actually a fault of the AF system in the body (user) rather than a limitation of the lens? For example, if I try and focus on a specific object or location with Lens A and it can't get focus, wouldn't I have the same problem with Lens B on the same body?

Snydremark wrote in post #9784642
When people complain about their lens hunting for focus, isn't this actually a fault of the AF system in the body (user) rather than a limitation of the lens? For example, if I try and focus on a specific object or location with Lens A and it can't get focus, wouldn't I have the same problem with Lens B on the same body?

The AF system in teh camera needs to be able to "see" a distinct feature in order to do its job. If it is too dark or if you have the camera pointed at a completely blank scene like a painted featureless wall then it doesn't have anything to see. Note that this is the light coming through the lens so "too dark" can be as much a function of max aperture on the lens as it can be the actual ambient light level and remember that most Canon cameras cannot reliably focus unless the lens is atleast f/5.6. If you have the lens focussed on something that is very close and then you point it at a scene where everything is very far away then scene might be so blurred that from the AF systems point of view it is looking at a blank, featureless wall. Since this situation exists, if the AF system cannot see anything it will do a "focus search" telling the lens to focus all the way in then all the way out and will keep looking for something to focus on while the lens is doing this. If the lens has a very long travel between close and far and/or if the lens has a focus drive system that moves very slow then this process can take a very long time.

The focus search can be disabled in a custom setting in the camera which prevents the long wait while it hunts but also means that if you are in the situation of changing from very close to very far then it may not even try to focus.

Thanks Ken, that was my understanding; but I wanted to be sure. Since I discovered that the little 'focus confirmation' flash of the focus point works with manual focus too, I've just started turning off AF when things are just too dark

Snydremark wrote in post #9784805
Thanks Ken, that was my understanding; but I wanted to be sure. Since I discovered that the little 'focus confirmation' flash of the focus point works with manual focus too, I've just started turning off AF when things are just too dark

The AF system either has enough light to decide the focus or not. If the AF system has enough light to detect the focus and give a confirmation flash then by definition it has enough light to function and should be able to AF normally. All you're doing is using your hand instead of the focus motor.