The ‘Video Input’ tab at the top opens the Video Input dialog:

This helper dialog enables you to check that a video input device, such as a camera, is working correctly.
1. Select your Video Input Device. Once selected you can click the ‘Settings’ button to change the device settings.
2. Select you Audio Input Device.
3. Select a compatible Video Input Resolution of your device you would like to display.
4. Select whether you would prefer to keep the aspect ratio of the resolution selected or whether you would prefer to use the resolution of full screen mode.
5. Double clicking on the video makes the output full screen to simulate real playback.