Just when I thought I had almost every focal length covered, here comes the Sigma 18-35 f/1.8 (for APS-C cameras). Right now I have the Sigma 35 1.4. I like the focal length on FX. On DX, it is a normal lens, which is ok, though I prefer wider angles. The Sigma 18-35 1.8 fits that need exactly. It has a prime-like wide aperture for the shallow DOF that I like, and it covers focal lengths that I find very useful: ~28mm, 35mm and 50mm equivalents.
Of course, because this is a zoom, one question is what its image quality is like. No one knows yet, but a clue is the MTF chart released by Sigma. First, here is Sigma's MTF chart for the Sigma 35 1.4 (one of the sharpest, if not the sharpest, lenses at its focal length):
Now here is the MTF chart for the 18-35:
The comparison is not fair because the Sigma 35's MTF chart is for full frame, whereas the Sigma 18-35 is for APS-C. Nonetheless, at 18mm the Sigma 18-35's sharpness and contrast on APS-C is similar to, if not better than, that of the Sigma 35 on full frame. At 35mm the edge-to-edge sharpness on APS-C is significantly better than the Sigma 35 1.4 on full frame. So much so that if you compare the 18-35 to the Sigma 35 1.4 at APS-C size, the Sigma 18-35 looks a bit better(!!!). Just a reminder: the Sigma 35 1.4 is a prime lens, and one of the sharpest lenses at that. Is this for real? I will be the first in line to get this lens if the price is justifiable! Sigma please don't overprice this lens, I'm begging you!
UPDATE: here is another MTF chart to look at. This is from the Sigma 17-50 OS, the sharpest standard zoom for Nikon DX (Canon mount also available) according to DXO.
Sigma's dedicated website for the 18-35 1.8: http://www.sigma-global.com/en/lenses/cas/product/art/a_18_35_18/index.html