While shooting yesterday (bikes on track) I found I had a hell of a lot of soft images quite suddenly. The good ones were soft, the poor ones dire, shooting in fairly bright sunlight at 1/400th (higher shutter speed than I would normally find necessary at that spot for any reasonable success rate). Using a Sigma 120-300 F2.8, and also a Sigma 70-200 and a Canon 100-400.
Landed up going an using my 20D instead.
No settings I can find that I have altered by mistake. Camera had been focusing fine shortly before at far shorter distances on bikes moving far more quickly relative to the camera. 3 different lenses had no effect, and worked OK with the 20D. No sudden changes in exposure that might indicate a sticking shutter (~140K on the shutter).
However, any suggestions on how to nail down the issue? What would be appropriate for varifying that the AF system is working (especially in AI Servo mode)?
All the best