Distagon T21mm f/2.8
This 21mm f/2.8 lens achieves its best DxOMark Score at f/2.8. If its resolution is not as high as many other Zeiss lenses, it remains solid both on full-frame and on APS-C sensors. Its most noticeable advantages are, here again, rather limited distortion for a wide angle lens, and very few chromatic aberrations. As with many wide angle lenses, this 21mm proves a solid companion for landscape both on APS-C and 24x36 sensors, and can also be used for reportage when mounted on an APS-C body.
On a full-frame sensor (EOS 1Ds MkIII):
On a full-frame sensor the resolution map is very sharp from the center to the first one-third, even fully opened. Stopping down to f/4 makes the same center to one-third of field even crisper; but as soon one reaches f/5.6, even if the center remains crisp, the last third and the corners become very soft. The best aperture range for this lens is definitely located between f/2.8 and f/4.
Vignetting is quite pronounced at wide apertures, but at f/5.6 and onwards it becomes almost anecdotic.
On an APS-C sensor (EOS 7D)
The behavior of this 21mm mounted on an APS-C body is almost identical with its behavior on a full-frame, as far as definition is concerned. The center is correct fully opened. Stopping down to f/5.6 will produce a very sharp zone on two-third of the map, but the last one-third and the corners always remain much softer. Closing to f/8 and smaller reduces the size of the central sharp area. Here the best aperture range is located between f/4 (where the map is the the most regular) and f/5.6.
If vignetting is visible at f/2.8, it vanishes two steps further. As soons as one reaches f/5.6, the map in perfect.