I think the director was trying to save on lighting costs - impossible to see some scenes clearly. Artistic effects are all very well but it does help if the characters can be seen.
Efficient adaptation of the book - which is much better than the film - as is audio version.
Shame director over-adapted the scene where the cardinals walk back to the Sistine Chapel post bombing - the sight of cardinals in their red robes joining hands and singing would have been much more effective than the bland umbrella scene chosen.