Colleagues: I was in the middle of a shoot last night: simple circumstances, no trauma to camera, just a simple, not very physical event. Shooting 5DMkIII and my 70-200 II, on autofocus, not manual. Used this lens/camera setup a zillion times. I use "1/2 press shutter" focusing; I do not use back-button focusing. I was using the smallest focusing target, one square. Suddenly one square stayed highlighted (in black) towards the left of the viewfinder. The other single-point that I was manipulating continued to work. The camera focused on the movable square, NOT the single black square. I tried everything to fix this: turned camera off and on, checked all the switches on the lens, dismounted/remounted lens. Black square stayed put. I panicked. Finally I did the 'system reset' on the camera, returning all settings to the original, 'out of box' tabula rasa state. The black box disappeared, and now the screen is normal. Does anyone know what happened? I am quite worried--I use this about 6-7 days a week and don't deal with malfunctions well. Any advice gratefully received. Thank you for your time in reading this.
