The lens isn't fixed aperture. This is from his blog:
flaring is more apparent, thats nothing new, but it changes as you stop
down. At wide open, F5, nothing is particularly sharp, and there is
almost no contrast. Stop down to a F8, and suddenly its a new lens.
Razor sharp, contrasty, and beautiful. Come down to an F11 or
F16 and its suddenly hazy and soft again but this time with a far more
Link to blog: http://timurcivan.blogspot.com/2010/10/something-old-meets-something-new.html
He talks about aperture 2/3 of the way down