Aperture F Number (f-stop)
We’ve already taught that the aperture is measured in f-stops and that the smaller the number, the bigger the opening. The reason for this is because each measurement is actually the fraction of f/(number) IE f/1.4, f/2, f/2.8, f/4, f/5.6, etc.
The f in Aperture F Number simply stands for the lens’ focal length. That way no matter what the size of the lens, the f-number would be proportional or universally applied across all lenses. In other words, when using the aperture and shutter speed, using one lens will generate about the same exposure as using the same setting on a different lens.
As you can see in the illustration, the opening can have the same size of f/4, while having different physical measurements.
So the point of the f-number is just to convey the illuminance of the aperture opening. It’s done as a fraction of f because the physical measurement may change from lens to lens but the light it puts out will be the same, if the f-stop setting is the same.