The Live View implementation on the EOS 450D is in fact the third generation from Canon (if you consider the EOS-1D Mark III and EOS 40D as two iterations of development). This latest implementation provides both passive auto-focus (the typical mirror up, screen blank, auto focus, mirror down approach) as well as contrast-detect auto-focus which works in the same way as a compact digital camera, slower but doesn't require the mirror to lift or go blank (so useful / acceptable in certain circumstances). One annoyance with passive auto-focus in Live View still exists, that once AF has been achieved you have to release your finger from the shutter release button before the display is restored.
Live view display modes
Pressing the DISP button while in Live View toggles between the three available display modes, each with differing levels of overlaid information. The final view shown below is the default view with 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)|
|3: Live view with magnification area + status line+ overlay + live histogram||Optional 'thirds' grid lines overlay|
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 cursor buttons to move around the live image.
Live view Depth-of-Field preview
One very useful feature in Live View is 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. This was more effective on the 450D than the 40D at small apertures (the EOS 40D didn't boost the brightness of the live view image enough).
|Normal Live View||DOF preview button held at F36|
Live view AF video clips
Below you will find two short video clips, the first executing auto-focus (from infinity) in live view with the 'Quick mode' option; normal passive auto-focus which requires the mirror to drop and hence the display is black during AF and also the 'Live mode' option which performs auto-focus using a contrast-detect algorithm just like a compact camera, this is slower but doesn't require the display to blank. These sequences have been trimmed so that from pressing the play button on the clip there is exactly one second before the shutter release button is pressed.
Live view 'Quick mode' Auto Focus
This is obviously the fastest method of Auto Focus in Live View but ironically requires you to release your finger from the shutter release button once AF is achieved rather than automatically returning to Live View. This method took just under a second to focus (but the screen was blank for 1.4 seconds).
Live view 'Live mode' Auto Focus
Note that this method is quite a bit slower than 'Quick mode' (taking around 3.6 seconds here but up to 6 seconds in lower light conditions) but does have the advantage of not blanking the display and is theoretically at least more accurate.