Planar T 85mm f/1.4
The 85mm f/1.4 ZE has a remarkable light transmission capacity. But transmitting light is not everything, and this lens has the same problem on both APS-C and full-frame sensors: it is very soft at wider apertures—but only at widest ones, since stopping down a little bit changes everything.
This is rather well summed up by its DxOMark score, showing the best low-light performance of this lens at f/2.8. This will be the key aperture.
The rest is solid: good transmission, no distortion, little vignetting, and a reasonable amount of chromatic aberration contribute to making this 85mm a good candidate for portrait shooting and even for sports on an APS-C, where its focal becomes 136mm.
On a full-frame sensor (EOS 1Ds MkIII):
Everything changes at f/2.8. Opened wider than this, the lens produces a very soft image. Hardly 20lp/mm at f/1.4… this is quite poor. Stopping down to f/2 makes the center better, but still soft. Things seem dull… but when f/2.8 is reached, everything changes. The center is suddenly very sharp, around 60lp/mm, the first one-third is solid, and the edges are almost good. Between f/4 and f/5.6, the whole first one-third is very good, and the rest of the field map stands above 50lp/mm. By the way, this area is the best one for this lens. Stopped down to f/8 and above softens the center, but maintains the field around 50lp/mm.
On an APS-C sensor (EOS 7D)
On an APS-C sensor, the scenario is the same. Wider than f/2.8, the lens is not sharp enough. At f/2.8 it suddenly becomes sharp all over the field map, and remains so until stopped down to f/8. Precise and regular.
Vignetting is noticeable, although very light when the lens is fully opened, but vanishes when stopped down to f/2.
This 85mm achieves its best DxOMark low-light score at f/2.8 on both APS-C and full-frame sensors, and its best mid-light score at f/4. Sharp, it produces almost no distortion, limited vignetting, and limited chromatic aberration on the edges of the field.
This is a good companion for portraits, and even for sports, thanks to the crop factor that an APS-C sensor provides.
On a full-frame sensor (D3X):
As with the 50mm we studied earlier, the maximum aperture lacks definition on the whole field and the lens has to be stopped down by 2 stops to offer a central zone above 50lp/mm. At f/4, the center becomes very good, and at f/5.6, the sides of the field rise above 50lp/mm as well.
Vignetting, visible at wide apertures, completely disappears at f/2.8 and smaller apertures.
On an APS-C sensor (D300s):
The resolution scenario is very similar here. Very soft when wide open, the lens has to be stopped down to f/4 to reach both its best resolution and to achieve good homogeneity on the whole field.
Already very limited at full aperture, vignetting completely vanishes 1 stop later. At f/2, there is no visible vignetting.