Is the Nikon 17-55 DX on a digital Nikon really that much better than the "film" 17-35 because of its digital optimisation? What exactly does this optimisation do, besides, of course, narrowing the field of view to the smaller "APS" sensor? Since I shoot both film and digital, I'm really wondering if the 20 mm of added focal lenght are really worth the trade- off of not being able to use the lens on a film body. Your opinion, please. Thanks.