Hi Frank,


The 17-55mm is preferred for it's constant f/2.8 aperture, yes - that's a big advantage for moving subjects in low light and for creating a desirable portrait-like out-of-focus background blur. (Side note - the 17-55mm has a significant image quality advantage over the predecessor of the 15-85mm lens, the now-discontinued 17-85mm lens; but the IQ of the 15-85 is reportedly very good). Above 38mm, the 15-85mm is already at f/5, which is 1 2/3 stops slower than the 17-55mm, and for decent OOF blur at f/5 the background needs to be a good distance behind your subject.


Personally, I don't 'miss' the 55-85mm part of the range. But then, I've got that covered as long as I'm willing to mount a different lens. If I only had the 17/18-55mm range, I'd certainly want something longer. I would miss the constant f/2.8 aperture, a lot. But that's pretty dependent on what and how you're shooting.


The 15-85mm looks like a very nice lens, especially for tripod-mounted shots or when paired with an external flash. If you plan to shoot indoors with it, I'd definitely recommend also getting an external flash like the 430EX II. As long as you don't have an aversion to 3rd party lenses, the Tamron seems like a good choice, too.