My F100's center focus spot seems to be nearly blind. I have started noticing many situations in which one of the outer sensors (usually the right or left ones) can lock focus easily, but the center sensor, when placed over the exact same part of the scene, will hunt several times and sometimes refuse to lock focus at all. I've tested this with nice, high-contrast (of both brightness and focus depth) targets, in all kinds of lighting, and the behavior seems to be consistent. I have tested it with a 20-35/2.8 AF-D and a 50/1.4 AF with the same results, as well as a manual-focus 300/4.5 Ai lensso it's definitely the body. Is this a known issue? Can it be calibrated somehow, or is this indicative of a failing autofocus sensor module?