Canon EOS 5D Mark III Instruction Manual (en)

If you turn on the power switch and the Date/Time/Zone setting 
screen appears, see page 36 to set the Date/Time/Zone.
<1> : The camera turns on.
<2> : The camera is turned off and 
does not operate. Set to this 
position when not using the 
Whenever you set the power switch 
to <1> or <2>, sensor cleaning 
will be executed automatically. (A 
small sound may be heard.) During 
the sensor cleaning, the LCD monitor 
will display <f>.
You can still shoot during sensor 
cleaning by pressing the shutter 
button halfway (p.44) to stop the 
sensor cleaning and take a picture.
If you repeatedly turn the power switch <1>/<2> at a short 
interval, the <f> icon may not be displayed. This is normal and 
not a problem.
To save battery power, the camera turns off automatically after 1 
minute of non-operation. To turn on the camera again, just press the 
shutter button halfway (p.44).
You can change the auto power-off time with [52: Auto power off
Turning on the Power
About the Automatic Self-Cleaning Sensor
 About Auto Power Off
If you set the power switch to <2> while an image is being recorded to 
the card, [Recording ...] will be displayed and the power will turn off after 
the card finishes recording the image.