If the camera is in the factory default "One Shot" mode, then it will not take a picture UNLESS it can confirm that it was able to lock focus on a subject. No focus... no photograph.
If, however, you flip that little switch on the side of your lens that says "AF/MF" to the "MF" position (that's "Manual Focus" and the AF is "Automatic Focus") THEN you get an exception... in the "MF" position the camera is willing to take a photograph regardless of whether it thinks the image is focused or not... it doesn't care. By flicking that little switch to the "MF" position you've told the camera that YOU will take care of the focus and it's not to consider focus at all.
If the camera WILL take a photograph (even if it's blurry) when the lens is in the "MF" position then it means the reason it wasn't taking a photo before is due to it's inability to focus.
There can be a few reasons why a camera might not focus.
1) Not enough light: To test this situation... just go to a place where there is plenty of light. Like... outside during the middle of the day.
2) A problem with one of the focus systems. It turns out your camera actually has two different (totally independent) focus systems. One is used when you are looking through the viewfinder to compose the shot. The other is used if you use the live-view screen to compose the shot. The reason there are two different systems is because your camera has a mirror inside. When you look through the viewfinder you are actually looking through the lens. To do this, they have to place a mirror between the lens and sensor so you can look through the same lens the camera is using. But that mirror blocks all light from reaching the sensor. So they actually a focus system that doesn't use the sensor and can focus when the mirror is in the way (the mirror actually bounces some of the light into the focus sensors). When the mirror is flipped out of the way (for live-view shooting) it obviously cannot flip any light into that special focusing sensor. So instead it uses the camera's imaging sensor to work out the focus.
The two systems are independent and the only thing in common is that they both send instructions to the lens to move the focus in or out. If one focus system isn't working... try the other (test focus looking through the viewfinder... and test focus by using "live view" mode (not looking through the viewfinder.)
3) But that brings us to the third possible problem: The lens. The focus motors (for any Canon camera) are in the lens... not in the camera. The camera merely sends instructions to the lens telling it which way to adjust the focus and how far. If there's a problem with the lens, then it can fail to focus... even if there's absolutely nothing wrong with the camera itself.
If you have a different lens, you can swap lenses and see if the camera still has a focus problem. If it only has a focus problem with one particular lens... it's the lens that needs service and not the camera.
Tim Campbell
5D III, 5D IV, 60Da
