Canon EOS 5D Mark III Instruction Manual page 41

Attaching a Lens Hood
When the dedicated EW-83H hood is attached to the EF 24-105mm f/
4L IS USM lens, it blocks stray light and protects the front of the lens
from snow, rain, dust, etc. Before storing the lens in a bag, etc., you can
attach the hood in reverse.
If the hood is not properly attached, it may obstruct the image's
periphery, making it look dark.
When attaching or detaching the lens hood, grasp the base of the hood
when turning it. Grasping the hood's edges when turning it may deform
the hood.
Align the red dots on the hood
1
and lens edges.
Turn the hood as shown in the
2
illustration.
Turn the lens hood clockwise until it
attaches securely.
Attaching and Detaching a Lens
41