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It is difficult to recreate this scenario in daylight, but I did test it a bit yesterday on a bright day and it didn't seem to back focus. I need to find something "stepped" to shoot that is sort of like that choir, but it doesn't seem to have the problem in real light. B
I've demonstrated the effect to myself with simply a paper target by the window, tripod mounted camera w/remote release. Shoot with daylight, close the blinds and shoot under tungsten lighting, and the focus is shifted. Fairly repeatable probably 70-80% of the time the fcous changes, but not 100%. Drives me crazy, but I calibrate my lenses to daylight and try not to shoot in tungsten lighting, or at least try to make adjustments if I do shoot in tungsten lighting. What's even worse is with some zooms, the degree of focus shift also seems to change with focal length. I think with the really high res cameras today it's more easily observable; not quite sure if it's technology that's gone bad.
Another possibility is that the focusing area is larger than the face you are focusing on, and it's capturing the bearded man the lady behind it since that has more contrast. In that particular pciture, I probably would have checked focus in playback, and maybe aimed the focus target at the hand surrounded by the black music book.