Canon EOS 5D MARK III Instruction Manual page 26

.#.2: Custom Controls
When you press the depth-of-field preview button, the aperture wiii stop
down and you can check the depth of field (p.165).
With the lens' iS switch set to <ON>, 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 wiii
display a grid and an electronic level using the AF points.
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