Canon EOS 5D Mark III Instruction Manual (en)

82: Custom Controls
When you press the depth-of-field preview button, the aperture will stop 
down and you can check the depth of field (p.167).
With the lens’ IS switch set to <1>, the lens’ Image Stabilizer 
operates when you press the button assigned to this function.
When you press the button assigned to this function, the viewfinder will 
display a grid and an electronic level using the AF points.
: Depth-of-field preview
: IS start
: VF electronic level
Over 4°
Over 6°