Lello,
Just to keep you going while I rummage, Here is link to a test. Scroll to the bottom to ge to the test
http://www.cameralabs.com/features/18200_issue/
This second link ( an exrtact ) will help to gve some idea of the effect on performance of stopping the lens down. Note the softness at f5.6 compared to f16.
http://www.cameralabs.com/reviews/Ni...m/page4c.shtml
Don