Sigma 17-70mm F2.8-4 Contemporary

Aberrations

Imperfections in an optical design can lead to chromatic aberrations (such as fringing), lens flare, and ghosting.  When used with a lens hood, modern glass coatings do an excellent job of suppressing lens flare, and that's what we observed with the 17-70mm.  Thanks to its two super-low dispersion elements, the 17-70mm also minimizes fringing quite well.

This 100% crop of the center of a high-contrast scene shows that you will hardly ever notice any fringing in the center of the image:

Fringing (red and green) is only apparent in the corners of the image, where the lens produces the most distortion:

Still, this is a fairly small amount of fringing, and it will likely go unnoticed in a print or web upload.

To test for lens flare and ghosting, we pointed the camera directly at the sun and shot a picture to illustrate the worse-case performance.  In the 100% crop below, a green spot of ghosting can be found in the lower-left corner, but there is no flare:

Thus, we consider the new Sigma 17-70mm to control aberrations exceptionally well.  It is even better in this respect than its predecessors, which were already quite good.  It is also better than many of the constant F2.8 zooms that we have tested!