AF Techniques with EOS HD Video
   
Autofocus and Manual Focus
Autofocus — Still vs. Video Focusing
Auto focusing while shooting stills is completely different than when shooting video. This can be a challenge for photographers whose only experience is shooting stills, as well as for experienced videographers who have little experience with HD-SLRs.
Stills with Eye-level finder
AF System on the EOS 7D

When shooting still pictures, the camera
uses a sophisticated sensor that’s dedicated exclusively for autofocus operation. This “phase-detection” AF system can detect whether a subject is in or out of focus. More importantly, it can detect what direction to move the lens to get sharp focus; if the subject is moving; and how fast the subject is moving.

The phase detection AF system can quickly acquire focus and track moving subjects, but the mirror must be down for this system to operate. Therefore, you must view the scene through the eye-level finder when using the cameras primary autofocus system.
Video with LCD monitor
Live Mode

Quick Mode
When shooting video you cannot use the
eye-level finder — you must use ‘Live-View’
and look at the LCD monitor on the back of
the camera.

When Live View is active, the mirror is up,
therefore the camera must read focus off the
actual imaging sensor. This is called “Live
Mode AF”.

Alternatively, the camera can stop recording, get out of Live View, lower the mirror, and let the actual AF sensor acquire focus. This method of focus is called “Quick Mode AF”.
 
 
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