Quote Originally Posted by Daniel Browning
For example, people often lament that there is no FF equivalent to the 17-55 f/2.8 IS. But there is, and it's even better! The only reason they didn't see it is because they were held back by their f-number-centric frame of mind. The 24-105 has lower light intensity per area (exposure) but it has so much larger area that it more than makes up for it and actually collects more total light and has less total noise. You actually have to stop down the 24-105 by a third stop to get images similar to the 17-55.

Does this mean that a 16-35/2.8L on a crop body will also have to be stopped down to F4.5 to get the same exposure as with the EF-S 17-55/2.8 at F2.8 on the same body?