The DigiCam is mostly connected via the FireWire port or with new models about the USB port with the computer.
A remote control of zoom, screen and focus by software on the computer is not possible according to my knowledge.
If one sees the live picture on the monitor, it can be taken up immediately. Alternatively one is able to do also exposures on the memory medium of the camera memory. These often have a better quality than the exposures of the live picture. While taking up on the memory medium of the camera, one should absolutely use the remote control of the camera so as not to move the camera.
The FireWire stack in the Kernel has, e.g., in Fedora from 12 to 14 problems with the DigiCam which I use for the development (Canon MV5). These were repaired in Fedora 15. One can see this in the debug messages if one sticks the camera to the FireWire port.