This tells me your camera is unable to confirm focus.
When your camera is using "One Shot" mode, it is ALSO using a feature called "focus priority" (focus priority and one-shot mode go hand in hand). Focus priority means that the most important priority for shutter release is that the image must first be focused. If you completely press the shutter button, the camera will insist on focusing BEFORE the shutter opens to take the photo. Most of the time that's what you want.
But there's a counter notion of something called the "decisive moment" -- this is the idea that when shooting an image, there's a moment where everything in the scene is exactly as you want (particularly in "action" photography). It just wouldn't do to be pressing the shutter button and having the camera take the shot a half second later.
The camera has "AI Servo" mode which is intended for taking photos of subjects whose focus distance is constantly changing -- typical of "action" photography. When you put the camera into "AI Servo" mode, it uses a feature called "release priority". That means the camera WILL take the photo when the shutter button is completely pressed and it will do so EVEN if the camera hasn't focused (whereas One Shot mode using focus priority will refuse to take the shot until it can confirm it has focused.)
BTW, AI Focus mode is a kind of "pseudo" mode. Really the camera only has One Shot behavior and AI Servo behavior. AI Focus is a kind of "smart" mode that tries to auto-detect which of the two modes it should use. It beings by locking focus on a subject and then monitoring for a very short period (1/2 sec) to see if the focus distance to the subject is changing. If the distance does NOT change it treats it as a One Shot situation. If the distance DOES change then it treats it as an AI Servo situation. The problem with AI Focus mode is that it takes the camera a moment to decide which mode to use -- and when you're trying to capture the "decisive moment", that delay can cost you the shot you really wanted.
So now you know about focus priority and release priority and it should make sense why a camera will take the shot in AI Servo and wont take it in One Shot mode (btw, you'll find it also takes the shot if you set the lens to manual focus mode.)
The question is WHY isn't the camera focusing?
Usually it's the amount of light which is to blame. A camera that wont focus in dim lighting situations may work fine when outdoors in plenty of light.
Sometimes the lens' focus system is failing and not responding to the camera (although the camera can report an error message when that happens and you didn't indicate that your camera is throwing an error.)
Some lenses naturally collect more light than others. For example, the EF 50mm f/1.4 lens collects SIXTEEN TIMES more light than the 18-55mm lens (when using the 50mm focal length) using the lowest focal ratio available on both lenses. That gives that 50mm lens a HUGE advantage when trying to photograph subjects in poor lighting.
Tim Campbell
5D III, 5D IV, 60Da
