Canon MARK III EOS-1 DS Instruction Manual page 191

Shooting
No images can be shot or recorded.
The memory card is not properly inserted. (p.32)
If the memory card is full, replace the card or delete unnecessary
images to make room.
Or if a not-full memory card is inserted in the other card slot, switch the
recording to that card. (p.32, 73, 128)
If you try to focus in the One-Shot AF mode while the focus
confirmation light <o> in the viewfinder blinks, a picture cannot be
taken. Press the shutter button halfway again to focus, or focus
manually. (p.36, 86)
The LCD monitor does not display a clear image.
If dust is adhering to the LCD monitor, wipe with a lens cloth or soft
cloth.
In low or high temperatures, the LCD monitor display may seem slow
or it might look black. It will return to normal at room temperature.
The image is out of focus.
On the lens, set the focus mode switch to <f>. (p.35)
To prevent camera shake, hold the camera still and press the shutter
button gently. (p.36, 40)
The memory card is unusable.
If a memory card error message is displayed, see page 48 or 193.
Troubleshooting Guide
191