The Mamiya Sekor CS 3.5/14mm Fisheye lens shares its optical design with the eralier Mamiya Sekor SX 3.5/14mm Fisheye.
According to Roland Stauber, the CS 3.5/14mm Fisheye is oerforming rather well even at f3.5, but its best at f8 to f11:

http://mamiya-nc-m42.mflenses.com/cs_14_3.5.htm

 

 

 

Mamiya CS 14mm f35 Fisheye lens section
 		 MAMIYA SEKOR CS 14mm 1:3.5 Fisheye                 
(10 Linsen / 7 Glieder)