Canon f/2.8 lenses and focusing speeds

[Curtis Cunningham]

I have heard that if you have a 2.8 lens, something gets "turned on" in your camera body that increases the speed with which the lens focuses.

Are there specific Canon bodies that this only applies to?

[Will_Mass]

Canon 40D Interactive Review

[Curtis Cunningham]

What about a Digital Rebel XT? Does this body have the capability to focus fast with a 2.8 lens?

[ChrisPerry]

when using 2.8 and faster lenses an extra center sensor is turned on that makes focusing faster and more precise.
It applies to 30D and older XXd and XXXd bodies. the 40D is different, as are the 1D bodies. I'm not sure where the 5D falls in this.

[Dennis_Vied]

The 5D manual states that when a lens with 2.8 or larger is mounted, "high precision" cross-type sensing is available with the center sensor. With a lens of 5.6 or smaller, only cross-type sensing is available with the center sensor. (Nothing about between 2.8 and 5.6). Other sensors are either horizontal or vertical sensing.

Nothing about extra sensors, except in the case of AI Servo AF, where 6 invisible sensors kick in.

[Ronald Garrett]

I think I read either in a review or in the manual somewhere, {?} that the 40D has the feature to use the 2.8 speed to kick in an extra sensor. From experience I know the 300mm F/4.0 lens is slightly slower than the 400mm F/2.8 I just got. When the 400 is already close to focus, as say when going from focusing on one deer to another when they are standing side by side, the focus change is instanteanous to the point I can't tell it has changed except that I can see that it has. I get the same performance from my 70-200 F/2.8 Nikkor on my D2x.

What I am getting at is the quality and speed of the lens itself has a lot to do with focus speed. If the increase in focus speed when this extra sensor kicks in is say 1/100 second I doubt I would be able to tell the difference. Focus precision is another thing entirely.