The standard film rate is 24 frames per second. So when using the standard 180 degree shutter, the frame gets exposed for 1/48 of a second. (Just theoretically, if there was no shutter, one frame would be exposed for 1/24 of a second, but since there is a 180 degree shutter – blocking light half of the time – the frame gets exposed for only 1/48 of a second).