Like the EOS-1D Mark III, the EOS-1Ds Mark III features a fairly rudimentary live view system that doesn't support autofocus (of any kind - not even the mirror flipping kludge used by DLSRs without contrast-detect AF). What you do get is the ability to magnify any portion of the frame up to 10x and - more usefully - live view over USB when shooting in tethered mode (complete with remote manual focusing) or over Wi-Fi when using the optional WFT-E2.
Live View exposure simulation
You can optionally (via C.Fn IV-16) enable Live View exposure simulation mode where the Live View display will attempt to simulate the brightness of the final exposure. In this mode changes to exposure compensation or exposure (in manual mode) are simulated, and you can view the live histogram.
Live view display modes
Pressing the INFO button while in Live View toggles between the four available display modes, each with differing levels of overlaid information. Some of the screens below show the optional 'thirds' grid lines enabled.
|1: Live view with magnification area indicated + status line showing shutter speed, aperture, flash compensation, exposure compensation, frames remaining and ISO sensitivity.||2: Live view with magnification area + status line + brief overlay (simulation mode, battery status, Picture Style, AE lock, flash-ready). There is also an optional grid lines display, as shown here.|
|3: Live view with magnification area + status line+ overlay + live histogram. Note that the histogram is only available if you've turned on the Live View Exposure Simulation custom function.||4: Live view with magnification area (and nothing else)|
Live view magnification
Just as in playback mode you can magnify live view by pressing the enlarge button (or back out again with reduce). While magnified you can use the multi-controller to move around the live image. The Mark III appears to be able to go all the way to 1:1 magnification (one pixel on the sensor for one pixel on the LCD) which makes it easy to achieve perfect focus (it's worth noting that while magnified the camera applies higher sharpening than in the final image to assist focusing).
Live view Depth-of-Field preview
One very useful feature in Live View is of course depth-of-field (DOF) preview, when the DOF preview button is pressed the camera stops the lens down to the selected (or metered) aperture which provides you with an accurate representation of the depth-of-field of the final image. As you can see from the images below this worked well in our test scene at F11 (though in very low light and with very small apertures you will see the image getting darker). Note that unless you have live view exposure simulation turned off the screen will go very dark when the lens is closed down significantly using depth of field preview.
|Normal Live View||DOF preview button held at F11|