Center focal point working only with F/2.8 lenses

Richard Martin , Sep 06, 2010; 12:56 p.m.

I have a question that i can't seem to find a straight answer to; on the center focal point of the 40D, 50D, etc. it is said that with lenses of F2.8 or greater activates a more sensitive center focal point with cross/horizontal/vertical lines for more accuracy. I get this concept but what no one seems to have an answer to is what happens when you use the F/2.8 lens at smaller apertures? Does that make the more sensitive focal point revert back to only cross line sensitivity and how much of a difference has anyone noticed......I'm toying with changing equipment and would like a sure answer....Thanks in advance!

Responses

Geoff Sobering , Sep 06, 2010; 01:01 p.m.

The auto-focus system operates before the lens stops down for the exposure, so all that matters is the maximum aperture.

Puppy Face , Sep 06, 2010; 01:55 p.m.

AF is always performed at the maximum aperture of the lens. It stops down only when you take the picture. This has been standard since the popularization of the auto diaphragm (long before most of us were born).

Chuk Tang , Sep 06, 2010; 06:52 p.m.

What everyone else said. It will work fine as long as the max aperture of the lens is 2.8 or wider.