F value which means the brightness of the lens.Lens specifications shown in < maximum diameter ratio 1:1. 2 > of < 1.2 > is the F value.F value, the less said the lens of the higher brightness. F value from every narrow level, the brightness is tripled. The sectional area of the lens and the quantity of the incident light beam [lens the square of the effective diameter [D]] in proportion, and so the image brightness of F value square inverse ratio. The implied, the F value from every narrow level, brightness is tripled.