Sigma 28mm f/1.8 EX DG - £345/$450
The Sigma 28mm is designed first and foremost as a fast, wide-angle prime lens for full-frame cameras. As such, it's a big, heavy optic - the largest in the group by quite a margin. The lack of an internal autofocus motor isn't too much of a drawback, as all full-frame Nikon DSLRs have a screw drive for actuating autofocus directly from the camera body.
With an effective focal length of 42mm on APS-C format cameras, there's also the temptation to use this as a 'standard' prime lens on DX bodies. However, size and weight make the Sigma 28mm less than ideal. The camera-driven autofocus is a bit sluggish, but sharpness is great even at the maximum aperture of f/1.8.
This lens is a good choice for indoor, handheld shooting, with wide-angle potential on a full-frame body. However, there's noticeable vignetting at f/1.8, although this disappears at smaller apertures or when you use the lens on an APS-C format camera. Barrel distortion is noticeable and rather worse than that displayed by many of the other lenses in the group.
Sharpness is both impressive and consistent throughout the aperture range, only dropping off at f/16 to f/22.
Sharpness at f/2.8: 1905
Sharpness at f/8: 1920
Sharpness at f/16: 1691
Chromatic aberration is well contained, making the Sigma 28mm a good choice for landscape shoots on full-frame cameras.
Fringing at f/2.8: 0.19
Fringing at f/8: 0.26
Fringing at f/16: 0.21
The second-worst lens on test in terms of distortion, there's a clear barrel effect. However, it's still better than most zooms.
Image test verdict
Apart from some noticeable barrel distortion, the Sigma 28mm is optically very good, and is well suited to full-frame use.