If you use the viewfinder do you hear the motors attempt to focus and then give up?
You may have dust blocking the focus sensors inside the camera. Your camera has two different focusing systems.
When you use the camera by looking through the viewfinder, the reflex mirror in the camera is in the "down" position so it can bounce light up into the screen in the viewfinder. But there's a section in the mirror that allows some light to pass through and then bounces it down into sensors that are on the floor of the camera. If the secondary mirror is damaged or if anything is blocking the tiny holes that let light into the focus sensors then the camera would not be able to focus.
When you switch to live-view, the reflex mirror swings up and the shutter opens. Since the mirror is now folded out of the way, it cannot bounce light down into the normal focus sensors. So instead it relies on something called "contrast detection" using the camera's image sensor to determine when you've achieved optimal focus. This focus system isn't nearly as fast and it usually has to do a bit of trial and error to refine focus (the normal focus sensors on the floor of the camera are much faster). But since this uses a completely different method to focus, it explains why you are still able to focus using "live view" mode.
Something might be blocking the focus sensors (e.g. dust). And if that's the case, the camera might just need a good cleaning. But it's also possible that either the focus system has failed or the secondary mirror is sticking and not correctly bouncing light into the focus sensors -- and that would mean the camera needs service.
Tim Campbell
5D III, 5D IV, 60Da
