By default the HD stream on the 5mp cameras come set to there maximum supported resolution which is 2592x1944, this will give you a 4:3 ratio image. The SD stream will by default be set to a 16:9 resolution output.
If you would like to set the cameras HD stream back to a 16:9 aspect ratio, it is recommend to set the cameras 'Encoder 1' resolution to 2304x1296. Setting the camera to this resolution will make the camera broadcast a 16:9 aspect ratio image.
The access the video settings below, log in to the camera and navigate the following menus:
Setup --> Advance Mode --> Video/Audio