Whenever I start getting lens happy and start looking at the pro level mid-range zooms with a constant 2.8, i.e the 17-55 2.8 or the 24-75 2.8 nikons, I often read the the zoom range is not very useful on dx format as these are meant for full frames. What then are the optimal zoom ranges, and do I need to factor in the 1.5 crop factor when looking at these lenses, as in is the 17-55 really at 24-83?