Lens speed

The lens speed is depend on a size of aperture (f number or f/value) by f/value can calculate from ratio of focal length and aperture's dimension. For example, aperture size is 35mm and focal length is 50 so the f/value is 50/35 = 1.4 and the lens speed is 1.4 also.
The lens speed will show on the font of lens as:

High dimension of aperture this make f/value is smaller and more light go inside the lens so it is higher lens speed. Compare between f=1.2 and f=2.8 we can said that f =1.2 is more speed than f =2.8