The SMC Pentax-FA 31mm f/1.8 AL Limited is a full-frame moderate wide-angle lens for Pentax K-mount.

An earlier design by Jun Hirakawa of a smaller 31mm f/2.4 lens was rejected by management for being too "slow" (i.e. too narrow at maximum aperture), and so the f/1.8 version came to be developed by Takayuki Ito and Masayuki Murata.

[1] Popular Photography said in its March 2002 issue that the Pentax 31mm Limited was one of the greatest prime lenses they had ever tested.

[2] Photozone called it "an excellent lens",[3] and ePhotozone noted, "If we compare this 31mm with Leica prices, then we have tremendous value for money".

[4] In a Pentax Lens Tournament held in 2014, the users of pentaxforums.com, one of the biggest Pentax-related online communities, voted the 31mm Limited to be the Greatest Normal Pentax lens[5] as well as the Second Greatest Pentax Lens of All Time after the FA 77mm Limited.