Canon EOS 5D Mark III Instruction Manual (en)

 Using the Self-timer
If you will not look through the viewfinder when you press the shutter button, 
attach the eyepiece cover (p.187). If stray light enters the viewfinder when 
the picture is taken, it may throw off the exposure.
The <k> enables you to shoot while not touching the camera 
mounted on a tripod. This prevents camera shake while you shoot still 
lifes or bulb exposures.
After taking self-timer shots, playing back the image (p.250) to check 
focus and exposure is recommended.
When using the self-timer to shoot only yourself, use focus lock (p.67) on 
an object at about the same distance as where you will stand.
To cancel the self-timer after it starts, press the <o> button.