Stimulates retinal guanylyl cyclase when free calcium ions concentration is low and inhibits guanylyl cyclase when free calcium ions concentration is elevated (PubMed:7520254, PubMed:8626484, PubMed:9651312, PubMed:26703466). This Ca(2+)-sensitive regulation of retinal guanylyl cyclase is a key event in recovery of the dark state of rod photoreceptors following light exposure (PubMed:7520254, PubMed:8626484). May be involved in cone photoreceptor light response and recovery of response in bright light (By similarity). Homodimer. Subcellular location is not affected by light or dark conditions. Detected in the retina (PubMed:7520254). Detected in rod and cone photoreceptor cells (at protein level) (PubMed:8626484, PubMed:9620085). Also present in certain pinealocytes (PubMed:10821676). Binds three calcium ions (via EF-hands 2, 3 and 4) when calcium levels are high. Binds Mg(2+) when calcium levels are low.