>>4216636the 2.8 is just the ratio between the focal length of the lens and the width of the glass.
Just from a quick google, if you want to calculate the f-number of a given lens yourself do N=f/D Where f is the focal length, D is the lens diameter of the lens aperture and N is the calculated f-number.
With the pictured lens you can use this same equation to workout the diameter of the lens by re-arranging the equation to be D = f/N which would be 26 / 2.8 and give you a diameter of approximately 9mm
If you wanted a 100mm at f/2 then you would do 100 / 2 which would mean the lens would have a aperture diameter of 50mm.
If your eyes glazed over and you actually meant "yeah but how can a diddy lens be as good as a bigguh lens" then it's down to glass/engineering quality and also sometimes whether the lens is a zoom or a prime as not all zooms can maintain a constant aperture throughout
