Those of us who use compact digital cameras appreciate the benefits of the LCD that lets us compose a scene as well as watch exposure changes as they happen. Live View LCDs on digital single lens reflex cameras work differently.
Currently, not all DSLRs have a Live View LCD. The first did not appear until 2006, when Olympus introduced Live View on the EVOLT E-330.
With DSLR Live View, the LCD blacks out
When using Live View, the DSLR mirror is lifted first so that the autofocus sensor can be used in a similar fashion to Point-and-Shoot. However, unlike a Point-and-Shoot, there is a short period where the screen blacks out. The amount of time varies between a fraction of a second to several seconds, depending on the brightness of a scene. This process happens every time you refocus.
The momentary blacking out may not be a problem for photographing stationary subjects or manually focusing, but a black screen is less than ideal for photographing action. In addition, DSLRs are heavy, so holding one at arm’s length to compose and take a shot can be downright difficult. Using Live View for framing is most suitable when a tripod or other camera support is used.
Dual auto focus modes in Live View
The Canon Rebel XSi I’m considering offers the option to switch in Live View from phase detection autofocus to contrast detection autofocus. The latter is the autofocus method used in compact digital cameras.
Contrast detection AF is very accurate but a bit slower than phase detection AF. However, the screen doesn’t black out when using contrast detection. On the XSi, contrast detection AF can be made the default for Live View.