I'm perplexed...why does this happen?

jseivert wrote in post #5612167
Hey Boomer, can you let us know if this is a rare deal or if it's a constant problem?

I guess this is not exactly rare, because this shot was taken shortly after the first and although no PP was done it shows better focusing than the first... my guess is that the high contrast in the center background screwed up the first one. What do you think?

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If I were to hazard a guess, maybe the AF was "hunting" when you fired off the shot. In low light conditions or low constrasty subjects, the AF may need more time to focus, and in the first shot there may have not been enough image contrast to focus on (the shirt). With the woman in the picture, there now is enough of image contrast (curve of her nose) for the AF to work on. Of course, in this picture even with the right focus point, the man in front is out of focus. I guess the lesson to be learned is to keep everyone in the same "plane" when using large apertures.

Agree with the above. Pic #2 is well focused. Only thing causing the gent up front to be OOF is that he's outside the focal plane. A few things you can do to increase depth of field. Stop down to say f/5.6 - f/8, back up, or widen up (on a zoom lens.)

http://www.dofmaster.c​om/dofjs.html

Play around with the settings in the calculator to see just how shallow your dof is.

Did you have it set to One shot? or ai focus/ or ai servo??

Probably not One Shot. In that case the first image couldn't have been taken.
As said: DoF issue in the second, not yet focussed (or hunting) *and* DoF issue in the first.