Canon EOS 1DS MARK II 9443A009 Instruction Manual (en)

The shutter button has two stages. You can press the shutter button 
halfway. Then you can further press the shutter button completely. 
Pressing halfway 
This activates autofocusing (AF) and 
automatic exposure (AE) that sets the 
shutter speed and aperture. 
The exposure setting (shutter speed and 
aperture) is displayed on the top LCD 
panel and in the viewfinder. 
Pressing completely
This releases the shutter and takes the 
Shutter Button
If you press the shutter button halfway and (
) elapse, you must press it 
halfway again and wait a moment before pressing it completely to take a 
picture. If you press the shutter button completely without pressing it 
halfway first or if you press the shutter button halfway and then press it 
completely immediately, the camera will take a moment before it takes 
the picture. 
No matter what state the camera is in (image playback, menu operation, 
image recording, etc.), you can return to shooting instantly just by 
pressing the shutter button halfway.