picking autofocus points

skankmaster · 26 Nov 2006 at 10:18

Until now, ive not really given much thought about them and assumed everything within the boxes in the viewfinder were assessed - until I read about the horizontal and vertical orientation of the sensors for the points.

The autofocus points - generally I manually set just the centre point to be active, however, if I set it to automatic and it chooses a number of points that cover both near and far object contrasts, does it adjust the f number to find the correct depth of field to ensure everything is sharp between these distances? I know there is an A-Dep setting on the canon cameras, but does the same apply in Program/Av/Tv mode?

rpstewart · 26 Nov 2006 at 10:40

If you have multiple focus points active and the AF system decides to use more than one point then it'll focus at one distance, it may look like it has picked near and far objects but it hasn't. A-Dep is the only mode where the AF and the metering interact to pick an aperture based on a selected DoF, in the other modes the aperture is selected either manually (Av) or to give a correct exposure (P & Tv) with no consideration of DoF.

skankmaster · 26 Nov 2006 at 10:54

ah - I see. This then begs the question, are the auto selected points, ones which the camera thinks are roughly on the same plane or does it pick somewhere in between.

rpstewart · 26 Nov 2006 at 11:03

The AF system will initially go for the area of the frame with the most distinct contrast, if this area is covered by multiple points then it will use the extra points to confirm the focus. Any time multiple points are used they'll be in the same plane.

skankmaster · 26 Nov 2006 at 11:08

Thanks for the info - you're a wealth of knowledge!