To me it seems to be exactly the same thing. If sensor 1 is twice as big as sensor 2, iso 800 on sensor 1 has the same noise as iso 400 on sensor 2. So you can get the same shutter speed with the same noise with f/4 on the bigger sensor as f/2.8 on the smaller sensor.


You could say, "well, the difference is the sensor, not the lens. F/4 is f/4." But if the f/2.8 lens can't illuminate the bigger sensor, I think the terminology is appropriate. The 24-105 lens is actually capable of getting more light onto a ccd at f/4 than the 17-55 is at f/2.8. Similarly, I would call an f/4 medium format lens faster than an f/2.8 35mm one, because the medium format lens gets more light to the sensor (or film).


That's just terminology, though, and if you think I am crazy for using the word "faster" that is cool My main point is that I don't see any major disadvantage of the 24-105 f/4 on a full frame as compared to 17-55 f/2.8 on a 1.6 fovcf camera.