Auto focus v's manual selection of focus point

[swag72]

While out and about this weekend I was using the manual focus point selection on the joystick of my 50d. Works a treat, love it. Anyway, I happened upon a composition where there were 2 buds on either side of the pics. They were both over focus points, so I switched to auto focus selection to see what it would do.

Basically, both points on opposite sides lit up.

My question is this - If I was using a shallow DoF, and the buds were both at different distances from the camera, would they both be in focus? Or if not, how would the camera decide which point to take the focus from?

 

[Tobers]

No, and Yes.

No if you were at the widest aperture for the lens (e.g. in aperture mode). The camera would focus on one of them and the other would be out of focus. It would probably pick the closest one.

Yes if you were in P mode where the camera would probably work out what you were trying to do and set the aperture to give enough DoF to get both in focus.

 

[Canon Bob]

That's very different from my understanding of how the AF algorithm works.

Assuming that we're talking about One Shot AF.....
If multiple focus points are enabled then the closest point with reliable data will be used and any other points within the depth of focus (NOT depth of field) will be illuminated. There is no attempt to average the focus distance and only the image plane relating to chosen (by the AF s/w) point will be in sharp focus...although other points may well be in the same plane.

Focus is achieved with the lens wide open and there's no computation of depth of field whatever mode (TV, AV, P) you're using.

Bob

 

[swag72]

Thanks for that guys - Makes it clearer - Sorry for all the questions I keep asking!!