Anyone know the autofocus system and which point it favors in a composition?

I was just wondering, does anyone know what the algorithm of a 30D autofocus (all points enabled) is?

I usually like to use the center focus point but I am curious when composing, what does the autofocus like to do? pick a focus point on the closest object? ie I think digic III tries to use face detection and attempt to use those focus points? not sure...

Any ideas?

Thanks

The camera will generally grab the area of most contrast. With that being said the center point is the most sensitive focus point and therfore will tend to grab focus better than the other points if all are enabled.

The one farthest from the one you want. Seriously, I believe it favors the center focus point first.

Shoot a white wall at night with low light. Zero contrast, zero AF lock.

Watch what AF points light up as you track from the wall to a contrasted object, such as a wall plate or a picture frame.

Ive switched mine to single point AF, as it was always picking the wrong area (and definately not the centre in a lot of cases) on auto

Much better now, and I can move the AF around using the joystick if I need it off centre

cosworth wrote in post #3875774
Shoot a white wall at night with low light. Zero contrast, zero AF lock.

Watch what AF points light up as you track from the wall to a contrasted object, such as a wall plate or a picture frame.

After experimenting with it against my wall I now see how it works... I see the light now!

Interestly if i stayed in one spot where there are contrasty objects in say three of my points - if i just tried to refocus without moving it seems to randomly sometimes light up three, sometimes one ... sometimes two... or sometimes one focus point out of the original three that were locked.

What does it mean when multiple points light up at the same time. I always thought it mean those are in the same focus plane but these three objects were different distances from my camera.. maybe within 6 inches?

Foggy wrote in post #3875573
Any ideas?

The best idea I can give you is to select a single focus point, generally the one closest to your intended point of focus in your composition.

Auto AF point selection should be used only when tracking a moving target that's hard to keep centered in the viewfinder.

Your camera has several "features" that are designed for point-n-shooter who doesn't want to have to use his brain and his camera at the same time. They are not necessarily better than seemingly less advanced settings that allow you to control the camera.

Curtis N wrote in post #3877901
The best idea I can give you is to select a single focus point, generally the one closest to your intended point of focus in your composition.

Auto AF point selection should be used only when tracking a moving target that's hard to keep centered in the viewfinder.

Your camera has several "features" that are designed for point-n-shooter who doesn't want to have to use his brain and his camera at the same time. They are not necessarily better than seemingly less advanced settings that allow you to control the camera.

I've pretty much used only manual AF point selection via multidirection switch (when I make the shot). Sometimess I want someone to take a picture for me but before I do that I re-enable all AF points. I'm just wondering which AF points would be favored which has now been answered (thanks all!). Now I'm just trying to figure out what it means when more than one point lights up at the same time.

It's seemed to me that it tends to favor AF points on the closer subject. Which does put focus on the tips of people's noses fairly often. I'll switch the focus point manually when I have time, but often I'll rely on the automatic selection and recompose real quick if I notice focus lighting up on the completely wrong area. If it becomes too problematic, I'll switch to center AF point only and do focus-recompose.

My unsderstanding is that it is the AF point(s) at which a lock is possible for the nearest object that is(are) selected. Again, it is my understanding that currently face recognition is only implemented in cameras where the sensor is always exposed, as in P+S cameras. I doubt if it has been implemented via the Liveview function on the 40D, nor have I read or heard anything to suggest this feature has been implemented in the 40D. Had it been, I'm sure Canon would have made that quite clear.

BTW, the 30D has Digic II, not Digic III.

I never used auto selection on the 20D, I just found that it was almost never better than doing it myself. On the M2N the auto-selection has proved useful for some sports work, though I probably still a single point 80% of the time.