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Distagon T28mm f/2 

CANON MOUNT

This wide angle prime is one of the sharpest Zeiss lenses we tested. Its best DxOMark score (Low light – 150 Lux – 1/125th) is achieved at f/4 on both full-frame and APS-C sensors. This lens is a very good companion for reportage and landscape uses, and can even be suitable for portraits on an APS-C body. 
Sharp and balanced, this lens behave particularly well coupled with an APS-C sensor, where it produces slightly less chromatic aberration, distortion, and vignetting. But it remains a very strong performer on a full-frame sensor as well.

On a full-frame sensor (EOS 1Ds MkIII):

On a full-frame sensor, the resolution field map is above 50lp/mm on two-thirds of the field. The edges are a bit softer, but this is rather harmonious. Closing by 1 stop makes the center particularly sharp, but you have to close to 2 stops beyond, to f/5.6, to have both a sharp center and very good edges. At f/8, the very good central zone expands to the entire first one-third; but at f/11, the center becomes much softer. The best resolution is thus achieved between f/5.6 and f/8.
Vignetting is rather important fully opened, but almost forgotten at f/5.6 and beyond.

On an APS-C sensor (EOS 7D)

Mounted on an APS-C sensor, this 28mm proves quite sharp as soon as f/4, which is the aperture providing the best resolution on the whole map. Stopping down to f/5.6 and f/8 keeps the middle sharp, but smoothes the edges. F/11 and smaller apertures are not as sharp, losing up to 10lp/mm on the whole field map.
Vignetting handling is quite well here. Visible at full aperture, it completely vanishes at f/2.8.

Comparison:
Zeiss Distagon T 28mm f/2 ZE Canon v. Canon EF 16-35mm f/2.8L II USM

NIKON MOUNT

With a resolution reaching 60lp/mm on a D3x, this lens proves extremely sharp.  As often on the Zeiss lenses the chromatic aberrations are limited to the outer fringe of the map, leaving a perfectly handled frame. In this case this is true on a full-frame sensor; on an APS-C body however, this LCA phenomenon is really discrete and imperceptible at f/4 and smaller apertures. 
This 28mm achieves its best DxOMark low-light performance at f/2.8 on both full-frame and APS-C sensors, which is good enough for a fast lens. The mid-light best performance is found at f/4.
This lens is thus very good for landscape shooting on a full-frame body as well as on an APS-C; on the latter, the crop factor helps to make it a very nice lens for reportage, too.

On a full-frame sensor (D3X):

Fully opened, the lens provides a good enough frame, mostly above 50lp/mm, except for the edges. At f/2.8, the middle climbs above 60lp/mm and at f/5.6, the central two-thirds of the field are sharp, while the edges climb above 50lp/mm.
Vignetting is important when the lens is opened wide, but stopping down gradually decreases the loss of light, and at f/5.6 the loss is already inferior to one-third EV. The pattern does not improve at smaller apertures.

On an APS-C sensor (D300s):

On an APS-C sensor, the full aperture provides a soft frame. Stopping down to f/4 improves the center a lot, and things remain constant until f/8 is reached, when the edges are softened a bit. F/8 is the upper limit if you are looking for the best resolution.
Vignetting is important here, too, when the lens is wide open, but as soon it is stopped down to f/2.8, only the corners are one-third EV darker. At f/4, vignetting is no longer visible.

Comparison 1:
Zeiss Distagon T28mm f/2.8 ZF2 v. Nikkor AF-S 17-35mm f/2.8D IF-ED

Comparison 2:
Zeiss Distagon T28mm f/2.8 ZF2 v. Nikkor AF-S 24mm f/1.4G ED