The value of the aperture (iris) of the camera is shown by the F value. (It differs from lower f value.)
The value of the brightest (aperture open) status of the lens is called the open F value.
The open F value is used as an indicator of the lens brightness.
(Lens with small open F value is said to be “bright lens”.)
The brightness is halved each time, the F value becomes √ 2 times.
When the F value is 2.0 is 1, the brightness is halved F2.8 (2.0 X √ 2 = 2.8).