How big is the aperture of SLR camera lens now? Why is it marked?

At present, there are many aperture standards for SLR lenses, such as the maximum fixed focus of 0.95, the common 1.8, 2.8, 3.5, 4.0 and so on. The minimum aperture is generally written in the manual and is rarely used, such as below 22.

The aperture of a lens is the ratio of focal length to aperture, which is generally expressed as 1:2.8 (for example). The ellipsis is 2.8 aperture. So the size of the aperture is inversely proportional to the value.

Zoom lenses generally have two aperture values, namely the maximum aperture at both ends of zoom, which is expressed as 1:3.5~5.6 (for example).