В Хелпе написанно:
Bump Mapping Appears Muted
Limitation
Maya and mental ray use different bump-mapping algorithms, resulting in significant image differences between the two renderers. In particular, bump maps in mental ray renders appear more muted than they do in Maya.
Workaround
Increasing the Bump Height setting by a factor of roughly 5 will produce a mental ray rendering that more closely matches the expected Maya result.
А как найти? Просто. Вызываешь в Хелпе Search и набираешь Bump. И читаешь . . .