conventional method:
cropping the image and droping out the pixels
(move the cursor over the animation to rotate it)

Panasonic GH1: the sensor is wider than standard 4/3
and always exploits the image circle
4:3 - 3:2 - 16:9

the ultimate, square shaped (imaginary) sensor:
16:9 - 3:2 - 4:3 - 3:4 - 2:3 - 9:16