The F-number indicates how much light is allowed into the camera based on the size of the lens opening. The F-number is a ratio of focal length to the diameter of the entrance pupil. If the focal length is 100 mm, and the diameter of the lens is 25 mm, then the F-number is 4. It is written f/4. The photographer must take into account shutter speed when adjusting F-number and F/. This number helps gauge the distance a subject must be from a flash, if using a flash.