Originally posted by NickC Nope, don't get it.
I can only guess where "I am loosing you"
I guess that you followed you own line of thinking rather than thinking thru my argument. Probably, it is this external meter thing which traps you.
Ok, let's give me another try then, taking your external meter into account.
As you have said, you can set the same aperture (as an f-stop, not diameter, here), ISO, and shutter speed to record an image, according to the readout of your external meter. I am with you here.
But with a FF camera, you will actually capture twice the light if you do so (the aperture is 1.5x the diameter or twice the surface and therefore collects twice as many photons out of the same vivible field of view). Because of this, the performance of an FF sensor at, say ISO400, is
IDENTICAL to the performance of an APS-C sensor at ISO200. More or less by definition. BTW, by performance I mean the signal to noise ratio, or amount of visible noise in a print.
Are you still with me?
Ok, then it is clear why you can safely double the ISO settings with FF (compared to APS-C) to obtain the exact same image quality as with APS-C. And if you do this, you end up with lenses having the same diamater (in mm), hence weight and cost.
I may still have failed to explain myself. Please give me feedback if you can agree. Of course, I could be wrong but I don't think so.