Originally posted by EricT It is generally accepted that once you get over ~3x zoom ratio, you have to start making more serious compromises...It's all about what compromises in size, weight, speed, image quality and perhaps most importantly price photographers are willing to make.
EricT puts it well. A 17-70mm is not technically impossible, but is basically not
practical. Notice how most F2.8 constant lenses have ~3x zoom ratios, (a16-50mm is ~3.1x, a 28-75mm is ~2.7x, a 50-135mm is 2.7x, a 70-200mm is ~2.9, etc) while the 17-70mm lens has a ratio of ~4.1x. To keep it at F2.8 over this ratio would drive up the complexity of the design, and a theoretical 17-70mm F2.8 would most likely be pretty
big and
heavy, due to the need for large front element and a lot of glass. This increase in glass and materials, would then in turn drive up the
cost. So while I also would also love a 17-70mm F2.8, (the sigma 17-70mm F2.8-4.5 is my most used lens), I'm resigned to the fact that it isn't going to happen.