Autofocus Tutorial

[kevinbythebeach]

I was reading the autofocus tutorial section when i came across this section right under the interactive display showing focous points :

"Cameras used for left and right examples are the Canon 1D MkIII and Canon 400D, respectively.
For these cameras autofocus is not possible for apertures smaller than f/8.0 and f/5.6. "


What does it mean by not possible?

if I am reading the display and legend correctly , the larger the apeture the more 'crosses'
equals more accuracy=better focus????

sorry for my typos its 2:38 am and my insomnia is keeping me up.

[JonathanC]

What it means is that the autofocus systems will have a hard time working with lenses that are slower than f/8 and f/5.6. The reason is that you a)need light on the sensors and the slow lens will not get as much light, and b)the out of focus needs to be noticeable. With slow lenses like those mentioned, the depth of field is sufficient that the autofocus system can think that the lens is focussed when it isn't quite there.

[Colin Southern]

Keep in mind Kevin that they're talking about the MAXIMUM effective lens aperture here, not the currently selected aperture ...

eg: If I mount my EF70-200mm F2.8L IS USM lens on my camera and stop it down to F32, it'll still auto-focus because it does that with the lens wide-open at F2.8 - despite what you select, it only actually stops down to the selected aperture a few milliseconds before it takes the shot.

It's more of an issue with slow lenses (eg F5.6 max aperture) that people then start adding teleconverters to (at which point the Canon 1D series cameras will continue to AF 1 stop lower).

[kevinbythebeach]

Colin...ahhh ok....

[Zephyrize]

woah,
very interesting thread!
I haven't actually read about this issue
hmm, now it explains a lot of things..