I have been playing with a Canon camera with for me their latest AF facilities and have noticed some oddities just through the viewfinder. They may be down to using a Sigma lens along with a teleconverter but not sure yet.
One setting I have used to test for lack of focus is to set the 1st focus priority option so that the camera wont take a shot unless it reckons it's in focus. This proved for me that for some reason the camera itself was refusing to focus at times. The OP could try setting this and taking a number of test shots. If the camera takes the shot then focus should be ok plus nothing wrong with the shots.
AF can be out. Unlikely but it can happen. It's pretty easy to test for AF accuracy using these instructions.
http://www.northlight-images.co.uk/a...djustment.html
Notice the word ideally relating to test distances if that is a problem. Live view will always focus correctly so the final check involves comparing shots taken with normal AF with these. Even the preview screen on the camera will give a rough idea of accuracy.
In my case the next check is have Canon done a firmware upgrade on the camera as the behaviour does seem a little odd.
John
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