Tooffy- yours looks like it was optimized for use wide-open. I changed the shim on mine, added 0.1mm, to use at F1.5. Before that, it was best at F4 and compared poorly against the Nikkor-S 5cm F1.4. My personal opinion is that Canon optimized it for use stopped down, and that is why it never got the fame of the Nikon Sonnar.