Though it's not the first SLR to have a Live View mode, the Canon EOS 1Ds Mark III is only the second professional camera to have a true, full-color live view. (The first was its sibling, the EOS-1D Mark III). It's also the second SLR to allow both remote control and live view through a computer via USB, Ethernet, and WiFi (the latter two methods require a special adapter, the WFT-E2A). Note that many (it used to be all) Canon digicams and SLRs come with some form of remote control software that works via USB. Up to now, only the digicams offered live view through the computer.
See the illustration below (Courtesy of Canon USA) for the display and information available in Live View mode on the camera.
The EOS-1Ds Mark III's Live View mode has two options available in Setup Menu 2. They are Live View Shoot, and Grid Display. Live View Shoot simply turns Live View mode on. There is no dedicated button for this on the 1Ds Mark II, so turning it on is rather involved. You must press the Menu button, navigate to the Setup Menu 2, select Live View Function Settings, select Live View Shoot, then select Enable. The second option is to enable a grid that aids in composition, using the same menu. Live View mode also shows the selected aspect ratio, using bars. See the animation on the right.
Like the 1D Mark III and unlike more recent Canon SLRs, the EOS-1Ds Mark III does not support autofocus of any kind in Live View mode. Manual focus must be used, however you can magnify the Live View image by 5x or 10x using the Enlarge button, to assist in achieving critical focus. The focus frame, which indicates where the image will be magnified, can be moved freely around the screen using the multi-controller. See the animation at right. Interestingly, the lens can be remotely focused manually when connected to a computer running the included Canon software, via USB or WFT-E2A.
Another Live View option buried in the Custom menus (C.Fn IV-16) enables Exposure Simulation, where the camera will darken or lighten the preview to simulate the current exposure settings, as shown on the right. A live histogram overlay is also available by cycling through the four Live View display modes using the INFO button. Of course, depth-of-field preview is also supported in Live View mode.