Canon EF300mm F2.8L IS USM Instructions Manual page 16

11. Hood
The special hoods that come with all these
lenses cut out unwanted light and protect the
front of the lens from rain, snow, and dust.
To attach the hood, loosen the hood lock knob
by turning it counterclockwise. Fit the hood onto
the lens's hood mount, and tighten the lock knob
to fix it in place. Use the same procedure, in
reverse, to remove the hood.
12. Case
When putting the lens into its case, reverse the
lens hood, slip it over the lens, and attach the
lens cap. Then position the tripod mount straight
upwards and lay the lens into the case so that it
fits snugly into the compartment.
When carrying the lens, fasten the black straps
inside the case to hold the lens in place, and lock
the case.
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