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Distagon T18mm f/3.5

CANON MOUNT

This very-wide-angle prime lens reaches its best DxOMark Score (150 Lux - 1/60th exposure time) and its best mid-light scores (5000 Lux – 1/125th) at f/5.6 in both cases. Its very wide angles combined with its good performance make it a suitable lens for landscape and architecture shooting. Mounted on an APS-C body, the crop factor of the sensor makes this 18mm a bit longer (28.8mm) and suitable for reportage as well as for landscape shooting.

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

This 18mm delivers a very good resolution with 51lp/mm, and a rather restrained distortion measured at 0.4%, which is quite good for such a wide-angle lens. Its biggest weaknesses is  light transmission above 4 T-stops (certainly derived from its complex optical formula), and quite pronounced LCA limited to the sides of the map between f/3.5 and f/5.6, but invading a much bigger area at f/8 and higher.
The best resolution is achieved between f/3.5 and f/11. Wide open, its center is already very sharp. Stopping down to f/5.6 delivers a highly detailed central zone (up to one-third of the map), with the next one-third being very good too (above 50lp/mm). Stopping down to f/11 will make the last one-third of the map and the angles much better.
Vignetting is quite important wide open, mostly in the corners. At f/5.6, things are much better, but no matter the aperture, the vignetting is always noticeable.

On an APS-C sensor (EOS 7D)

Mounted on an APS-C sensor, the behavior is similar. The resolution decreases a bit, which is normal. Although the map looks similar to that of the full-frame sensors, the best definition is found between f/5.6 and f/8, where it remains above 50lp/mm on most of the map, except for on the edges; but the overall homogeneity is quite good.
Reducing the sensor size always has a very strong impact on vignetting. If it was important on a 24x36 sensor, on this sensor it is halved, and much easier to handle even at full aperture. At f/5.6, vignetting simply disappears.

Comparison:
Zeiss Distagon T 18mm f/3.5 ZE Canon v. Canon EF 16-35mm f/2.8L II USM

NIKON MOUNT

Mounted on a Nikon body, this lens achieves its best low-light and mid-light scores at f/4, except for the mid-light score on an APS-C sensor (achieved at f/5.6).
The strength of this wide-angle lens is clearly its reduced distortion (0,7% on a full-frame and 0,5% on an APS-C); on the other hand, chromatic aberrations are pronounced, especially on the last one-third of the field when mounted on an APS-C sensor, and its resolution may not be as high as expected.
 
On a full-frame sensor (D3X):

Associated with a full-frame sensor, this 18mm provides a sharp central zone at full aperture. Stopping down to f/5.6 makes this central area wider, making the whole first one-third clear and sharp. But the corners remain much softer. At f/8 they are degraded, but improve again at f/11.
Vignetting is quite visible at f/3.5, but gradually diminishes when the lens is stopped down. But although it is easily manageable, it never totally disappears.

On an APS-C sensor (D300s):

On an APS-C body, this wide-angle lens is not as good as it was on full-frame. When fully opened, most of the field map is soft. Stopping down to f/5.6 and f/8, and even f/11, makes the center much better, but the last one-third of the map remains softer.
Vignetting here is not a real problem. Noticeable at full aperture, it almost disappears when stopped down to f/5.6, leaving only the corners a bit darker.

Comparison:
Zeiss Distagon T18mm f/3.5 ZF2 v. Nikkor AF-S 17-35mm f/2.8D IF-ED