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.)
See individual images at: http://www.pbase.com/wjshaheen/fratio_myth__indeed
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 )
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