Canon EOS 5D Instruction Manual page 224

Troubleshooting Guide
The access lamp still blinks even when the power switch is
set to <2>.
If the power is cut off while an image is being recorded to the card, the
access lamp will light/blink for a few sec. When the image recording is
completed, the power will turn off automatically.
The battery becomes exhausted quickly.
Use a fully-charged battery (p.24).
The battery performance might be degraded. See the [7 Battery
info.] menu to check the battery's performance level (p.212). If the
battery performance is poor, replace the battery with a new one.
If you keep displaying the Quick Control screen (p.38) or shooting with
Live View function (p.107) for a prolonged period, the number of
possible shots will decrease.
The camera turns off by itself.
Auto power off is in effect. If you do not want auto power off to take
effect, set the [5 Auto power off] menu to [Off].
Shooting-Related Problems
The lens cannot be attached.
The camera cannot be used with EF-S lenses (p.31).
The card cannot be used.
If a card error message is displayed, see page 30 or 231.
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