This is a great telephoto lens and star testing shows a good behaviour
even near full aperture. Testing was made on a clear and cold night of
December 2010, with automatic tracking of my Kenko NES mount. Exposures
and ISO were selected to maintain a constant nominal exposure, except
the two higher apertures.
The results show that a good star shape is obtained even at f/3.5, and
that for very good puntiformity f/4.5 is best.
The focus test shows that reaching a perfect focus is a difficult task:
only after many trials a good focus was reached. Maybe a more
demultiplicated foculs control would have helped.
Thanks to Alessandro Gambaro for testing together his objective.
Aperture testing with Canon EOS 5D (full
Click on the image for the full-res
version (200% of original resolution)
Focus testing with Canon EOS 5D (full
Click on the image for the full-res version