As cone-specific cGMP phosphodiesterase, it plays an essential role in light detection and cone phototransduction by rapidly decreasing intracellular levels of cGMP. 3',5'-cyclic GMP + H2O = GMP + H(+) Binds 2 divalent metal cations per subunit. Site 1 may preferentially bind zinc ions, while site 2 has a preference for magnesium and/or manganese ions. Composed of two alpha' subunits that are associated with 3 smaller proteins of 11, 13, and 15 kDa. Belongs to the cyclic nucleotide phosphodiesterase family.