The Zeiss makroplanar 50/2 field is not especially flat (bent to the rear ad mid distances, iIRC), which is surprizing for a macro lens. On the other hand, marco shooter usually stop down.
The Tamron 45/1.8 may be what you are looking for, but I haven't tested it - only used it, to my delight (it has been sharing my Df with the 105/2.5 since I bought it). Its very homogeneous (and good) resolution wide open suggests a very flat field indeed - no "sombrebro" effect with that one.
Otherwise check the 50/1.8G, a very good overall performer, and much cheaper.