Canon EOS 5D Mark III Instruction Manual page 365

The access lamp still blinks even when the power switch is
set to <2>.
If the power is turned off while an image is being recorded to the card,
the access lamp will remain on/continue to blink for a few seconds.
When the image recording is completed, the power will turn off
automatically.
The battery becomes exhausted quickly.
Use a fully-charged battery pack (p.28).
The battery performance may have degraded. See [53: Battery info.]
to check the battery's recharge performance level (p.344). If the
battery performance is poor, replace the battery pack with a new one.
The number of possible shots will decrease with any of the following
operations:
• Pressing the shutter button halfway for a prolonged period.
• Often activating only the AF without taking a picture.
• Using the lens' Image Stabilizer.
• Using the LCD monitor often.
• Continuing Live View shooting or movie shooting for a prolonged
period.
The camera turns off by itself.
Auto power off is in effect. If you do not want auto power off to take
effect, set [52: Auto power off] to [Disable] (p.55).
Even if [52: Auto power off] is set to [Disable], the LCD monitor will
still turn off after the camera is left idle for 30 min. (The camera's
power does not turn off.)
Shooting-Related Problems
The lens cannot be attached.
The camera cannot be used with EF-S or EF-M lenses (p.39).
Troubleshooting Guide
365