New Photo shooting of Golshifteh Farahani behind the scene of a movie

A light meter works by taking the data you enter, including the ISO, shutter speed and framerate, and then by taking a reading it will tell you what F stop you should be at for the correct exposure. This doesn’t mean you need to set you F stop to that number, but what it might mean is you need to increase or decrease your lighting in order to achieve your desired aperture.