(in
Camera Lens News No. 10) the bulge increases with time after winding medium format film: it's small at 5 minutes, significant at 15 minutes and maximum after 2 hours. One solid piece of useful information from Zeiss: the bulge is only half as much for 220 film as it is for 120. The Zeiss rule of thumb is, " For best sharpness in medium format, prefer 220 type roll film and run it through the camera rather quickly." Temperature and humidity probably also affect flatness.