The DFA-50 f/2.8 or any of the F/FA/DFA-100 f/2.8 lenses are very good for this purpose. I recently helped a guy locally who did not want to go DSLR with digitizing his entire slide collection of around 4000 high quality 35mm color slides from safaris trips into Botswana, and he used his FA-100 f/2.8 macro on my K-3 with very good results. I did process testing with my DFA-50 and this also produced excellent sharp results. Result was equivalent to excellent quality scan at about 4800dpi with no field distortion or lack of sharpness.
He was lucky to be able to loan a color balanced light table for backlighting, and had a sturdy mini tripod for the setup. About 6 or 7 cans of dust-off and a month latter, he was done...! (BTW: for the testing I improvised a crude light table with a cool-white CFL 15W globe, and it worked fairly well.)
I suspect the Sigma 70/105 or Tamron 90 macros will also be very good options. The full 1:1 macro capability is not required because of the smaller sensor on APS-C compared to 35mm negative. But you will need 1:1 capable lens practically.
Edit: couple of other tips: camera on full manual and AF with multi-points enabled. The AF missed focus when on spot if no clear color/contrast in spot field-of-view. Shuttle on 2s mirror-up delay to eliminate any vibration with the relatively long shuttle speeds used (typically iso100, f/8, 1/6-0.5s range)
Last edited by KevinR; 04-25-2015 at 10:54 AM.