If the camera is in "One Shot" mode (it's default mode) then it also uses something called "Focus Priority". This means that it _must_ confirm that it was able to achieve focus before it will take a photo.
The other mode is called "AI Servo" which uses something called "Release Priority". In that mode the camera _will_ take a photo when you completely press the shutter button even if it was not able to focus (usually this is not a behavior you want unless you know how to watch for it to confirm focus, but sports photographers like this mode.)
There is a focus mode called "AI Focus" -- it's not truely a new mode... it really just means it will try to evalutate if a subject is moving or not and then try to use either "One Shot" mode or "AI Servo" mode depending on if the subject was stationary or moving.
ALSO... the camera will only attempt to auto-focus is the AF/MF switch on your lens is in the AF (auto-focus) position. If you switch it to MF (manual focus) then the camera should take a photo immediately (even if it's not focused).
Anyway... the camera can struggle to focus if the lighting is poor and/or if the subject lacks contrast (e.g. if you simply point the camera at a clear blue sky then it will actually struggle to focus -- even a starry night sky is a problem because the individual stars just aren't big enough and bright enough.
If your camera is having a problem AND you are taking a photo in poor lighting or of a subject that lacks contrast (or both) then this isn't actually a problem with the camera (even a perfect working camera would have problems in those situations).
It might help if we could eliminate the possibility that the reason the camera is struggling to focus is due to inadequate lighting.
Can you take the camera on on a nice birght sunny day and try to focus on a well-illuminated subject? This would allow you to either confirm or eliminate the possibility that lighting is to blame.
There is also the possibility that the lens is a problem. What lens are you using and do you have more than one lens?
If you have the standard "kit" lens that usually comes with a T3, it should be an EF-S 18-55mm f/3.5-5.6 IS II. This lens does _not_ allow for "full time" manual focus. This means you should not attempt to focus the lens if the lens is in "AF" mode. Switch the lens to "MF" mode. Turn the focus ring at the front of the lens all the way in one direction (it doesn't matter which way) until you have run the focus either all the way "out" or all the way "in". Then switch the focus mode switch back to "AF". Point the camera at a subject and attempt to focus. You *should* hear the focus motors on the lens as it attempts to focus. If you do not hear the focus motors, then it could indicate a problem with the lens. If, by some chance, you happen to own either a "USM" lens or the new "STM" lens, these lenses have focusing motors which are extremely quiet (espeically the "STM" lenses) so you likely would not hear the lens focus. But the "STM" version of the EF-S 18-55mm lens is extremely new and as it sounds like your camera is a few years old, you probably do not have the "STM" version.
Usually if the camera or lens has a malfunction then the camera would display an error message code. It doesn't sound like it's displaying a code, which makes me suspect it's not taking a photo because it's unable to lock focus (but the focusing mechanism is correctly attempting to focus). This is why I am wonderng if the reason it is not locking focus is due to inadequate light or due to a subject with inadequate contrast.
Tim Campbell
5D III, 5D IV, 60Da
