Full Screen / Pan-Scan Issue

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Hi, when I press 'F' to make the video full screen, there are two black bars on the left and right sides of the movie. This gets cleared up if I press 'O' (automatic pan/scan) twice, once to enable it then once to disable it, then I see the movie as it should be seen. However, if I exit full screen and press 'F' again, the bars are there again.

Is there something I can do to fix this? Thanks in advance.