I bought a 15-85 EF-S and am very pleased with its performance and have no intention of getting rid of it. I have been seriously contemplating purchasing a 17-55 EF-S, most specifically for the constant 2.8. However, in my head, I keep telling myself that increasing my ISO in the lower light situations (need for the faster 2.8 [though really only slightly faster]) will negate/eliminate any benefit provided by the 17-55 EF-S. Is my logic and reasoning sound? Before the questions fly, I am just an advanced amateur whose interests run the whole gamut of possibilities in what I photograph. Most of my photos/pictures are viewed only on a computer with a few printouts in the 5X7 and some in the 8 1/2 X 11 size. I don't do any poster sized pictures. I say this because of possible questions regarding digital noise from higher ISOs. Input would be greatly appreciated. I don't want this to be a Canon/Sigma/Tamron/Tokina thread. My question specifically is regarding the compensation with ISO on the 15-85 vs. the faster constant 2.8 of the 17-55. Thanks.