Sigma 16-28mm f2.8 Contemporary Review: Wide Whimsy, On The Cheap

Zooms can be wide, bright, or lightweight -- but rarely all three. That's the lure behind the Sigma 16-28mm f2.8 DG DN Contemporary, a wide aperture ultra-wide that doesn't add more than a pound to a camera bag. Has Sigma managed to design the trifecta of ultra-wide zooms? Or, is the downside written right there in the name as 16-28mm when most reach more as 16-35mm?