The 15-85mm is a good lens, provided you haveadequate light. If you're going to shoot low-light/indoors with available light, you're better off looking at one of the f/2.8 zooms, such as Canon's 17-55, or the Tamron 17-50. Personally, I want to replace my kit lens with both the 15-85 and the Tamron 17-50. One for it's large range for general outdoor use, and one for it'slower light requirements for indoor use, of if I need the thinner DOF.


Here's a comparison of the 15-85mm and your 28-135 on a 50D at 85mm. CLICKY. Mouse over the arrow, or the images to switch to the 28-135 lens. The 15-85 looks quite a bit sharper across the entire frame.