Distagon T25mm f/2.8
This lens is really sharp. But reading its metrics without digging in a bit more is misleading. At first sight one may think that the level of chromatic aberration is huge—25 µm is quite an important figure. But as explained earlier, its location on the field map makes it suddenly almost inconsequential. Nearly the whole field is perfect, except in the extreme corners, which are quite degraded. But this is really limited to a tiny external portion of the field, whereas when mounted on a APS-C sensor, this very same phenomenon occupies a bigger part of the frame, but still limited at 7 µm. Curiously enough, this time, 25 µm may be preferable to 7 µm.
The behavior of this lens, coupled with its focal length, make it a very good choice for landscape shooting and also for reportage when associated with an APS-C sensor with a 1.5x crop factor.
Its best DxOMark low-light performance score (150 Lux – 1/60th) is achieved at f/4, whereas for the best mid-light performance (5000 Lux – 1/125th), the lens has to be stopped down to f/5.6.
On a full-frame sensor (D3X):
Full aperture delivers a good central zone, representing a good first one-third of the frame. But the remaining portion lies between 20 and 30 lp/mm. Stopping down to f/5.6 makes this same area much sharper, and the second one-third of the frame quite good, standing above 50lp/mm. Only the corners remain soft. At f/11 these corners manage to reach 50lp/mm, although the central zone softens a bit (but still remaining quite sharp).
Vignetting is very pronounced a full aperture. Stopping down to f/5.6 and smaller improves things a lot, and only the last one-third of the map remains 1/3 EV darker, which is easy to compensate for.
On an APS-C sensor (D300s):
Full aperture delivers a very soft image, with the whole frame showing a resolution below 40lp/mm. Stopping down to f/4 improves things in the center, but going to f/5.6 and f/8 makes it even better with two-thirds of the field achieving above 50lp/mm. Closing to f/11 again reduces the resolution.
Vignetting is quite visible when the lens is fully opened, but completely disappears as soon as the lens is stopped down to f/4 or smaller apertures.