After reading of Stan Moore's "f-ratio myth"*, I decided to test the concept. And, I agree with him (that it is a myth). Afterall, when you adjust the f-ratio on a camera, all you're really doing is stopping down the lens - i.e., reducing the aperture. Makes sense - and I think this test supports his contention.

Both images below were taken with an SBIG ST-8300M (monochrome CCD camera) on an 11 inch Celestron EdgeHD SCT using 10 minute exposure times. (The images were taken as part of testing the Optec Lepus focal reducer and the f/10 was a baseline.)

If f-ratio determined "brightness", then the f/7 image should be twice as bright as the f/10. As it is, the difference is simply in the scale (and resolution) of the two images.

* ( See http://www.stanmooreastro.com/f_ratio_myth.htm )