f/4 is f/4. Period. A lens set to f/4 will let in a fixed amount of light, whether that lens is attached to a 1D4, a 5D2, a 7D, a T1i, or an old Elan film camera. The subsequent comments in that thread are best summed up by Daniel's statement, "Anyway, that doesn't take away from the importance of understanding what happens when the performance per area is the same and you vary the area." Key point there is we had moved on to a discussion of sensor characteristics. What comes out of the back of the lens is totally unaffected by that. To illustrate that point, you can compare any two sensors of the same size but different properties (e.g. 7D vs. 50D/T1i). Performance and light-gathering ability differ, independent of the lens.


There's no easy calculator illustrate this with amount of light, but there is with depth of field. Try the following experiment - go to the DoF calculator atDOFMaster. With the default settings on the page (50D et al. 1.6x crop body, 55mm, f/16, 10 feet from subject), you'll see a DoF of 6.61 ft. Change just the pop-up for the body to a 5D MkII, a full frame body. Now, all else being equal, the DoF is now 12.3 ft. What?!?!? Didn't we all agree you multiply the aperture by 1.6 for a crop, meaning the DoF will be thinner on FF? Why does changing the sensor from crop to FF make the DoF wider? Because of that clause about 'same framing' that I mentioned. Because of the crop factor, you are closer to the subject for a given subject framing on a FF body. With the DoF calculator, 5D MkII, 55mm, f/16, 6.25 feet from subject = 4.02 ft DoF; a ~1.6x aperture or f/25 gives a 7.44 ft DoF (the actual 1.6x would be f/25.6, but that's not an option). 6.25 ft FF framing = 10 ft framing on a 1.6x body. Settings of50D body, 55mm, f/16, 10 feet from subject gives 6.61 ft DoF, which is what f/25.6 on a FF would yield. So, for the same framing, same lens settings as above, a crop body gives a 6.61 ft. DoF, whereas a FF body gives a thinner DoF of 4.02 ft.


So, the bottom line is that a sensor does not and cannot affect the lens properties. f/4 is f/4. f/4 on a particular FF body may be 'brighter' than f/4 on a particular crop body, but then again, f/4 on a new FF body may be 'brighter' than f/4 on an old FF body. The light projected by the lens is the same, but how the sensor detects the light depends on the sensor.


And if that's not an egregious threadjack, I've never seen one... [:P]