Have a look here John
http://www.clarkvision.com/articles/...yth/index.html
This bloke Roger Clark knows what he is talking about and explains things very clearly. In fact his whole site is a mine of information.
One of the references he gives is also worth a look.
http://www.stanmooreastro.com/f_ratio_myth.htm
It seems the f-ratio myth was really due to the reciprocity failure of film. It just does not apply as much to modern detectors that have a far wider integration time than film.
Bert