One thing I have not seen noted, is the 40mm, as many said is a really good lens especially if the central or mustache distortion is corrected.
I must say though it is a lens that requires a lens shade, as it is quite prone to lens flare. With a shade no issues, but without, I have seen many users have an issue, including myself.
The issue with the 47mm is that it was not designed with the resolution of the 150mm back in mind either. Both the 40mm and 50mm will have better resolution than the 47mm and a 150mm back.
Hope this helps.