[Steve Smith]

It is my understanding that while the physical diameter of the aperture remains constant, the effective aperture changes from f4 to f8, two stops. And the light available at the film plane (because of diverging transmitted light rays coming from the open aperture area of the lens) is one fourth as much. This, I understand, agrees with the inverse square law.

I agree entirely with your understanding except that being English I would say a quarter instead of one fourth!

The f stop is easily explained by thinking of it in its simplest terms. i.e. focal length divided by aperture diameter. When you double the focal length and keep the aperture diameter the same then the f No. will double.


Steve.