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
frame)
Click on the image for the full-res
version (200% of original resolution)
Focus testing with Canon EOS 5D (full
frame)
Click on the image for the full-res version