In common with all new EOS DSLRs, the EOS 5D MkII offers a Live View mode. In Live view the reflex mirror is swung up and the shutter is opened, allowing the image formed by the lens to reach the digital sensor at all times. This image is then displayed in real time on the rear LCD of the EOS 5D MkII, just like on a "point and shoot" digicam.
In this mode there are two main auto focusing options (selected via a custom function). In the first focusing mode ("Quick mode"), the same AF sensors are used that are used in normal reflex mode. In order for focusing to occur though, the reflex mirror must swing down, which blocks off the image on the sensor. So the mirror swings down, the LCD image blanks out, the camera focuses the lens using the normal (fast and accurate) AF system, then the mirror swings back up and the now focused image is restored on the LCD. In the second focusing mode ("Live Mode") the image formed on the sensor is used to detect focus by maximizing contrast. In this mode the image never blanks out, the reflex mirror doesn't move and the shutter stays open. The disadvantage is that focus can be slow, it may hunt slightly and it may not be quite as accurate. It may also not always find good focus under difficult conditions (low light, low contrast subject, moving subject).
There is actually a third focus option which uses "Live Mode", but which looks for faces in the image. When it finds them it can optimize exposure and focus for the faces. This is called the "face detection" mode. In Face Detection Live mode, the largest face near center is detected initially, but the Multi-controller can be used to select any face detected.
Of course you can always use manual focus. The LCD image can be magnified by 5x or 10x in order to better determine the optimum focus point.
Closely associated with Live View is the movie mode. The EOS 5d MkII is the first Canon DSLR to offer the ability to record movies (such a function has long been available on most P&S digicams of course). To record movies the camera operates in Live View, but it can make a continuous recording of what is seen on the LCD screen in either 1920x1080 pixel 16:9 HD video or in 640x480 pixel 4:3 SD video, both at 30 frames/sec. The 5D MkII has a built in microphone on the front of the camera for recording mono sound, and there is a input jack for an external stereo microphone.
The maximum video file size is 4GB. This isn't so much a limitation of the camera as a limitation of the FAT32 file system which is used to store data on the CF memory card. This is the same file system used on most Windows PCs and 4GB is the maximum size allowed for any single data file in the FAT32 system. With the 5D MkII HD video, 4GB is enough to record for about 12 minutes. You can record any number of 4GB video files on a card, so you can record about 48 minutes of HD video on a 16GB CF card, but you have to do it in 12 minutes segments. In SD (640x480) mode you can record about 24 minutes in any one file.
Exposure in video mode is automatic, with the camera choosing the shutter speed, aperture and ISO and using center weighted metering. Exposure can be locked by pressing the "*" button (AE Lock). Shutter speeds between 1/25s and 1/125s are used. There is no continuous autofocus, but focus can be adjusted manually. Image stabilization operates as normal if an IS lens is used, but it's possible that the sound of the stabilization gyros may be picked up by the microphone in the camera if the external sound level is low. The audio is under AGC (automatic gain control), so that when things are quiet, the mic gain is increased which can lead to camera operation sounds being recorded. Autofocus during shooting is possible in one shot "Live Mode", but it may be slow and there may be some overshoot or hunting.
Videos are recorded as .MOV files (with MPEG-4 compression), and the sound is recorded using uncompressed linear PCM (pulse code modulation). One word of note here is that if you play back a full size 1920x1080 HD video .mov file on a Windows PC, it will default to playing via the Quicktime movie player. On many (if not most) systems, the video will not ply smoothly. This is not the fault of the camera and doesn't indicate a fault with the file. The PC (or more likely the video card) is simply choking on the amount of data which is trying to be played! It's also said that Quicktime isn't well optimized on the PC and works better on a Mac (which makes sense since Quicktime is an Apple product). However There is a way that I found to get smooth playback on a PC using the free open source “VLC media player” program available from http://www.videolan.org/. Note that once you have installed it on your system you may have you may have to do the following to set it up for smooth HD video playback.
The EOS 5D MkII has the ability to grab still frames at any time while you are shooting in movie mode, but the movie is interrupted while the camera records the full resolution still images, so this isn't something you'd want to do if you were trying to shoot continuous smooth video.