Canon focusing?

[Hardrock]

I know the Canon lenses have the focusing motor in the lense but what actualy does the focusing the lense or the camera. I ask because my rebel xt seems to focus slow at times with my 100mm 2.8 and 50mm 1.4
(both have the usm), And was wondering if that is because of my older camera?

 

[Big Mike]

It's the camera that houses the focus sensors & control. The camera then tells the lens/motor what to do.

So your camera likely won't focus as fast as something like a 50D and certainly not something like a 1D mkIII.

That being said, there are other factors...like the amount of light you are working in and the actual objects that you are trying to focus on.

 

[inTempus]

I know the Canon lenses have the focusing motor in the lense but what actualy does the focusing the lense or the camera. I ask because my rebel xt seems to focus slow at times with my 100mm 2.8 and 50mm 1.4
(both have the usm), And was wondering if that is because of my older camera?

The physical focusing is done in the lens body. A number of factors play into how fast the lens can achieve focus. Some lenses are faster than others. My 85L is a pooch in terms of focusing speed. My 24-70 moves quite quickly.

The body has the AF sensor meaning it determines the focus then passes instructions to the lens to achieve focus. Some bodies are faster than others. My 1D blows away my 5D in terms of how fast it can determine focus.

 

[Hardrock]

Thanks alot guys for the info! So its my body that takes time to focus not my glass. Usually I dont have any problems but sometimes when inside of my house the camera has a really hard time focusing and goes from one end of the focus meter to the other and wont focus. I was kind of hoping it was the body and not the glass.

 

[Big Mike]

Some lenses still focus faster than others.

For example, USM is faster than non-USM (and quieter as well). 'Ring' USM is faster than non-ring USM.
Bigger, heavier lenses have more physical mass to move for focusing, so it probably takes longer than a smaller lens.

but sometimes when inside of my house

Sounds like your issue has to do with the lack of light. A better camera body, with a more sensitive AF sensor, would make an improvement.