Isn't the K28/3.5 supposed to be even better than the M?
It can sound weird, but my example of K28/3.5 has 93 l/mm in the center wide open and reaches than 97 a stop further. For the corners its 64 and 65. I didn't believe my microscope.
cheers -- alkos at tlen pl http://onephoto.net/portfolio.php3?id_autora=17765
