The human eye perceives smooth motion above a certain frame rate. This is referred to as frames per second (fps). In the TV sector, 25 fps is the standard. For particularly fast movements, such as sports footage, a frame rate twice as high is preferable.
Video sequences shot at 50 fps and then slowed down to 25 fps create the slow motion effect," explains Nicolai Deutsch, filmmaker, Canon Video Ambassador and Academy Trainer. The rule of thumb for exposure time is double the frame rate. So at 25fps you should expose at 1/50 second, at 50fps at 1/100 second.