GO:0047017:   prostaglandin-F synthase activity
Catalysis of the reaction: NADP+ + (5Z,13E)-(15S)-9-alpha,11-alpha,15-trihydroxyprosta-5,13-dienoate = NADPH + H+ + (5Z,13E)-(15S)-9-alpha,15-dihydroxy-11-oxoprosta-5,13-dienoate.