Hi and welcome to Discussions,
taken from Apple Video Developer Note found here:
http://developer.apple.com/documentation/Hardware/Conceptual/HWTechVideo/Articles/Video_implementation.html#//apple_ref/doc/uid/TP40003994-SW741200331181
*External Display Modes*
The default display mode setting on the MacBook is extended desktop display. To toggle between the two modes, press the F7 key, or go to System Preferences>Displays>Arrangement.
A scaling function is available when the internal display and an external monitor are both operating and the mirror mode is selected. However, the external monitor could have black borders during mirroring, depending on the supported timings between the two displays and on the monitor’s selection algorithm. Black borders are not seen on VGA displays.
Both displays show full-sized images when the display resolution for the external monitor is set to the internal display’s native resolution: 1280 by 800. Both displays can operate with other resolution settings, but in mirror mode, one of the displays may be smaller than the full screen and have a black border around it. With the resolution for the external monitor set to less than 1280 by 800, the image on the internal display is smaller than its screen. For resolution settings larger than 1280 by 800, the image on the external monitor is smaller than its screen.
In mirror mode, the maximum size of the external display is 1280 by 800 at 60 Hz. In extended desktop mode, the maximum size of the external display is 1920 by 1200 at 60 Hz.
Regards
Stefan
