MarkV wrote in post #8285161
No, but the distance doesn't change just because you use a different AF point. In your example, it's impossible to have all people in the group perfectly in focus.
Not at all. Line them up in a nice line, focus on the one in center and they'll all be in focus.
But, you say, those out at the ends are farther away from the camera!
Yes, they are, but you made the same mistake in your thoughts as I did the first time I heard about this. You also forgot that good lenses are calibrated to make a plane, parallel to the sensor/film sharp, not a spherical bowl.
So that's the reason for why focus/recompose doesn't work well, if the distance change exceeds the DoF too much.
Apart from that, if you use evaluative metering, then the camera will bias the exposure on which focus points are active. But it does lock exposure (provided you use One Shot AF!) automatically, so chance are reasonable that you'll get the correct value anyway. Or you can meter with a different method and/or set the exposure manually.
The 40D/50D cameras are the only Canon cameras with cross-type AF sensors only. They also support cross-type center high-precision AF with lenses with f/2.8 or better.
The 5D/5D Mark II had linear focus points, except that the center point becomes a cross-type point when using lenses with f/2.8 or better. On the other hand the 5D has a more dense array of focus points in the center, with the six assist points, but unlike the 1D Mark III (19 linear/high-precision cross-type main points and 26 assist points), the assist points in the 5D Mark II only works when in Servo AF mode.
I've not used any 5D/5D Mark II more than a few images, so I'll not compare these to the 40D/50D. But I have used the 400D and 40D a lot, so I can compare these two. The 400D has an AF module that's identical to the one in the 30D, which in turn is about as in the 5D, but lacking the assist points. When using One Shot AF they should be reasonably equal.
The 40D is noticeably better than the 400D when it comes to autofocus. But, again, if extrapolating that observation into the 50D/5D Mark II comparison has any relevance, I don't know.
With the 40D, I often use one point when using One Shot AF. Since they are all cross type, it doesn't matter so much. When using Servo AF, I sometimes use all points, and hope the camera will be able to transfer focus tracking from the center point to a peripheral point if needed. Sometimes it works, sometimes not.
It's possible to set the external flash (580 EX II) to not fire the flash, only the AF assist beam, if you want it to.