I use a 5x4 LF camera with a 6x9 and 6x12 rollfilm backs, both of which I consider to be medium format. I would consider LF to cover any film sized 10cm x 10cm and above. This is a more satisfactory definition than defining LF as being sheet film. I have an adaptor which enables one to take two seperate photographs on a single sheet of 5x4 - photographs I would define as MF.