The camera uses a different focusing system in "live view" then is used when you use the viewfinder.
Your camera has 9 auto-focus sensors on the floor of the camera. You can put the camera in a mode where it uses all 9 focus points. Be aware that if the 9 points are on subjects of various distances, the point the camera will pick will be the one with the CLOSEST focusing distance.
You can also put the camera into a mode where YOU pick which of the 9 AF points it will use.
Page 85 of your owners manual will show you how to pick a specific AF point or put it in the mode where it uses all of them.
Those focus points will ONLY WORK if the lens attached has a focal ratio of f/5.6 or lower. If you are using a lens with a teleconverter, you have to multiple the focal ratio of the lens by the teleconverter factor. E.g. a variable f/3.5-5.6 zoom which is zoomed in a bit is using f/5.6. Put a 1.4x teleconverter on that lens and it becomes an f/8 lens (autofocus will NOT work at f/8) or put a 2x teleconverter on that lens and it becomes an f/11 (auofocus will definitely not work at f/11). A 1.4x teleconverter is only appropriate for f/4 or faster lenses. A 2x teleconverter is only appropriate for f/2.8 or faster lenses.
Something MAY be blocking some of the AF points in your camera. How clean is it? You can remove the lens, switch it on, use the menus to put it into manual sensor clean mode. This will cause the reflex mirror to flip up and expose the sensor... but it also exposes the 9 AF sensors on the floor of the camera (you wont see the sensors... you'll just see the tiny holes/ports in the bottom and the sensors are hiding under those. If you see any dust or debris in there, then that would be a problem. Just get a hand-squeezed blower (like a Giotto Rocket Blower... this is the bulb you squeeze with your hand to get puffs of air) and blow it out. Don't use compressed air cans. They can leave a residue and if you get residue on those sensors, you've got a big problem because you can't get to them without disassembly of the camera. NO COMPRESSED AIR CANS... EVER!
If you can't see anything visibly in the camera and you aren't using a teleconverter that gets you above f/5.6 and you've set the camera to auto-pick the AF point and it still can't focus... then you should contact Canon service for help because you've pretty much exhausted what you can do on your own.
Tim Campbell
5D III, 5D IV, 60Da
