Sorry Sejanus I should have said of similar focal length whether zoom or prime.
See here for info about Canon lenses especially MTF diagrams and check out the 85mm F1.8 about $600 vs the 85mm F1.2L costing thousands of dollars.
http://www.usa.canon.com/consumer/co...categoryid=152
The 85mm F1.2L is only slightly better at f/1.8 than the 85mm F1.8 is at f/1.8. At higher apertures the differences are almost indistiguishable.
By the way Canon Super Telephotos are not lenses they are in a class of their own. They have two ED elements and a Fluorite element that makes them super APO's.
Not many people know that Canon make the Fluorite elements for the upmarket APO telescopes. They know who they are!
Bert