Stimulates retinal guanylyl cyclase when free calcium ions concentration is low and inhibits guanylyl cyclase when free calcium ions concentration is elevated (By similarity). 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 (By similarity). May be involved in cone photoreceptor light response and recovery of response in bright light (PubMed:25673692). Homodimer. Subcellular location is not affected by light or dark conditions. In the retina, expressed in rod photoreceptors (at protein level) (PubMed:9620085). Expressed in cone photoreceptors (PubMed:9620085). Binds three calcium ions (via EF-hands 2, 3 and 4) when calcium levels are high. Binds Mg(2+) when calcium levels are low. Guca1a and Guca1b double knockout mice show an increase in response to light in dark-adapted cone photoreceptors (PubMed:25673692). Dark-adapted cone photoreceptors show a delayed response time and a delayed recovery time in response to light (PubMed:25673692). Guca1a, Guca1b and Rcvrn triple knockout mice show rod photoreceptors have a reduced current decay during light response (PubMed:29435986).