Autofocus Question...

PaulieC

Hiya everyone!

I was reading a mag over the weekend, a review of the Canon 40D and I came across something I didn't quite understand. Had a look at the press release and this is what it says:

"The EOS 40D camera's completely redesigned nine-point AF sensor provides cross-type AF measurement at all nine focusing points for maximum apertures up to f/5.6, and for the first time in any EOS camera, the central AF point offers enhanced precision for both vertical and horizontal subject contrast when using EF or EF-S lenses featuring maximum apertures of f/2.8 or faster."

Not sure what this really means. Does it mean that it will only work on Lens that are f2.8 or faster? Will it not autofocus on an f4.0 lens? Generally, how does autofocus work? Is it based on contrast?


thanks

Fajitas!

AF indeed works on contrast, the system in place on the 40D and similar will work on lenses of all apertures, but it offers a more accurate focusing when using lenses that are f/2.8 or faster, purely because they leave more light in to focus on. It's a system in place on the 5D and 1D series too.

PaulieC

I see, it just works better on faster lenses? Handy that, since they are more expensive:rolleyes:

Does that also mean that AF will not work on a very slow lens, such as an f8 Mirrow lens?

Paulw

Here's some useful info to read - http://cpn.canon-europe.com/content/infobank/autofocus.do

Fajitas!

As long as the lens has an AF system that connects with the EOS system, they'll autofocus, but it'll be slower if it's at f/8 due to not as much light getting in.

PaulieC

Thanks Al, Paul. That's cleared things up a bit and put my mind at ease again!

Fajitas!

Any time