>Focusing on a subject's eye is a much easier target for the >AF system as it usually presents a high contrast area - the >centre of the eye is normally much darker than the surrounding >skin and in any case has a dark centre surrounded by white. >Your banana was a low contrast target and with lighting levels >that require ISO 1000 and f/1.4, there is very little contrast >variation for the AF system to detect. > >I've shot portraits with my 50mm at f/1.4 - f/2.0 and got the >eyes bang in focus very easily. Depth of field is so shallow >at these settings, that the ears and nose are out of focus.
That makes me think this really was about the lens. I took several hundred portrait shots with it and had very few usable results.
I don't think the banana was soft (heh, heh ) because the AF couldn't "see" it. I think it was soft because the AF was off.