Canon EF600mm f/4L IS III USM Instructions Manual page 5

General Precautions
Handling Precautions
If the lens is taken from a cold environment into a warm
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one, condensation may develop on the lens surface and
internal parts. To prevent condensation in this case, first
put the lens into an airtight plastic bag before taking it
from a cold to warm environment. Then take out the lens
after it has warmed gradually. Do the same when taking
the lens from a warm environment into a cold one.
Please also read any lens related handling precautions
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listed in your camera's instruction manual.
Shooting Precautions
The lens' focusing ring is electronic.
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The lens will not focus when the focusing ring is turned
while the camera is turned off.
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