I would recommend this lens, it is very sharp at wide open with slightly blur at the corner, and already extremely sharp entire the frame at f4.
see big file here..http://www.pbase.com/chounws3/testing_zeiss_25_f28_zm
|at f4. iso 200, 1/1250s|
|at f2.8, iso 200, 1/2000s|
|at f2.8, iso 200, 1/800|
|at f2.8 iso 200, 1/4000s|
|not sure which f stop been used, iso 200, 1/800s|
|not sure which f stop been used, iso 200 1/320s|