All EOS cameras feature three different focusing modes. Choose the one which suits your subject.
One-Shot AF:best for still subjects, such as landscapes and portraits.
When you partially depress the shutter button, the lens focuses and the focus is then locked. The AF points that achieve the focus will display in red and a green focus confirmation light will appear in the viewfinder. If the camera is unable to find the focus, e.g. your subject is too close, the focus confirmation light will blink and you will not be able to fire the shutter.
AI Servo AF:best for moving subjects when the focusing distance is changing.
When you partially depress the shutter button, the lens focuses on your subject. If you keep the button partially depressed, the lens will continually refocus if the subject moves and the focus distance changes. Focus tracking will continue for as long as your subject is covered by an active AF Point.
AI Focus AF:best for subjects moving unpredictably, such as animals.
The camera automatically switches from One-Shot AF to AI Servo AF if it detects your subject moving.
In low light, an AF-assist beam can help with accurate focusing. On cameras with built-in flash this is done by a series of flashes and is effective up to approximately 4 metres. Alternatively, a Speedlite or Speedlite Transmitter ST-E2 can be used which emit the AF-assist beam as a grid of red light. Subjects should be 10 metres or less away depending on the Speedlite.