Canon EOS 5Ds Instruction Manual page 473

Shooting-Related Problems
The lens cannot be attached.
The camera cannot be used with EF-S or EF-M lenses (p.52).
The viewfinder is dark.
Install a recharged battery in the camera (p.40).
No images can be shot or recorded.
Make sure the card is properly inserted (p.43).
If you are using an SD card, slide the card's write-protect switch to the
Write/Erase setting (p.43).
If the card is full, replace the card or delete unnecessary images to
make space (p.43, 358).
If you try to focus in the One-Shot AF mode and the focus indicator
<o> in the viewfinder blinks, a picture cannot be taken. Press the
shutter button halfway again to refocus automatically, or focus
manually (p.55, 141).
The card cannot be used.
If a card error message is displayed, see page 46 or 487.
I cannot lock the focus and recompose the shot.
Set the AF operation to One-Shot AF (p.87). Focus lock is not possible
in the AI Servo AF, or when servo takes effect in AI Focus AF (p.83).
Troubleshooting Guide
473