terribly stupid question

why won't xxD bodies focus with a lens over 5.6 when you can autofocus with a lens with, say a 2.8, stopped down past 5.6?

J-dubya wrote in post #8059156
why won't xxD bodies focus with a lens over 5.6 when you can autofocus with a lens with, say a 2.8, stopped down past 5.6?

Because focusing is always done wide open. The lens will only be stopped down when you press the shutter release button, otherwise it stays wide open.

On a side note, the DOF button on your camera can 'manually' stop down the lens to the selected F number. Try it, you'll see how the viewfinder gets darker as you increase the F number.

but they can AF with the bigma, which is a 6.3 right?

J-dubya wrote in post #8059341
but they can AF with the bigma, which is a 6.3 right?

the bigma lies to the camera about its maximum aperture. yes it will af but the cameras af mechanism is running (slightly) out of spec with that lens hence its iffy low light performance.

10-roger...thanks

All Canon EOS camera do AF and meter while the lens is wide open. So no matter what aperture you set to, they you AF at maximum aperture.

Now why xxD won't AF with lens with max aperture of f5.6 and small, there isn't any reason. I guess Canon just need to separate its 1D series from lower end body. 1D series will do AF with lens of max aperture f8. For lens like Sigma and Tamron superzoon, they have max aperture of f6.3 at its long end, the lens actually fool the body thinking it is at f5.6. So it is NOT xxD AF sensor cannot do AF at lens slower than F5.6, just the software of the camera disable the feature.

eelnoraa wrote in post #8063574
Now why xxD won't AF with lens with max aperture of f5.6 and small, there isn't any reason. I guess Canon just need to separate its 1D series from lower end body. 1D series will do AF with lens of max aperture f8. For lens like Sigma and Tamron superzoon, they have max aperture of f6.3 at its long end, the lens actually fool the body thinking it is at f5.6. So it is NOT xxD AF sensor cannot do AF at lens slower than F5.6, just the software of the camera disable the feature.

The AF does not work at f/5.6 and suddenly fail at f/5.7. Under bright, full sunlight my 40D can autofocus down to f/8, maybe even smaller if the subject offers enough contrast. In a poorly lit room the camera might have problems focusing even with a lens f/2 or larger. The engineers programmed the camera to stop trying to focus if the aperture is 5.6 because that's the cutoff under most lighing conditions. Lenses like the bigma can do their little trick to fool the camera but they also are known to have trouble focusing in poor light.

The same conditions apply to the AF system on the 1 series cameras too. f/8 is the cutoff under most lighting conditions but I bet that in full daylight they could focus at f/11 or maybe even smaller.