SIGMA DP3 Quattro - Manual (Page 40)

40
SHUTTER BUTTON
The shutter button of the dp3 Quattro cameras has two positions. When
you press the shutter button “halfway down”, the camera’s autofocus
systems will activate. When pressing the button “all-the-way down”, the
shutter will release to take a picture.
TIP

Before you start to take pictures, we recommend that you operate the
camera, to become familiar with the “halfway down” position and other
features of the camera.

Normally, if the card is not inserted in to the camera body, it is not
possible to release the shutter. If you would like to release the shutter
without a card, please set from [ Camera Settings] (P.27) → [Release
without Card].
Disable
(Default)
When the card is not inserted in the camera body, it is
not possible to release the shutter.
Enable
It is possible to release the shutter, even if the card is
not inserted in the camera body.