If auto-focus is enabled and the camera is in the default "One Shot" focus mode, then the camera will not take a photo unless it can confirm that it has locked focus.
Reasons that it might not be able to lock focus could be:
1) it's too dark (try shooting in an extremely well-lit situation... such as outside in the middle of the day (which definately qualifies as having enough light to focus.))
2) Subject lacks contrast (this is probably not the issue -- but the focus system does need something with contrast to lock focus. E.g. if you attempt to focus on the plain blue sky then it wont lock focus. But almost any real subject will usually have enough contrast to focus.)
3) Lens auto-focus is not working (e.g. a fault in the lens.)
In theory, there could be a problem with the auto-focus system within the camera itself, but that's extremely unlikely. The most likely culprit (assuming enough light) is the lens.
Do you have a different lens you can put on the camera to test?
Tim Campbell
5D III, 5D IV, 60Da
