F Number (F/#)

“F-number represents the brightness of the lens at infinity. The lower the number, the brighter it is.
The F-number is determined by the ratio of the focal length to the effective diameter of the lens.
FNO = f/D Effective Aperture = Focal Length / Entrance Pupil Diameter
As shown in the table below, the exposure is halved by changing F/# by one level.”