Yes, it has a broader focal range. If that were all that mattered, we'd all be using the Tamron 18-270mm on APS-C, right? Why would anyone pay close to $2K for the 85mm f/1.2 - it's only got one focal length! </sarcasm>Originally Posted by JTPAIN
There are three factors to consider between the two: focal length, aperture, and IQ. As you stated, the 15-85 wins on focal length. The aperture goes to the 17-55mm. The variable aperture of the 15-85 changes like this:
15-17mm = f/3.5
18-26mm = f/4.0
27-37mm = f/4.5
38-60mm = f/5.0
61-85mm = f/5.6
So, by 18mm it's letting in only half as much light, and by 38mm it's 1.67 stops slower. That means less ability to stop action, and less subject isolation (the 17-55mm is good for portraits, the 15-85 much less so). The IQ is close, but the edge goes to the 17-55mm - it has less distortion (broader zoom range means more distortion), and less vignetting (despite the wider aperture). Sharpness is similar, but while distortion and vignetting can be corrected in post, there's a cost for that - reduced sharpness and increased noise.
I usually recommend the 17-55mm over the 15-85mm because it's more versatile. If you will do almost all of your shooting outside in good light, or don't mind using a flash indoors (and not the popup, please!), the 15-85mm is a good option. The 17-55mm allows stopping action in less light and delivers shallower DoF for portraits, which IMO makes it more versatile.
Reply With Quote