The ab initio shielding calculations are carried out to investigate the conformation dependence of C-13 chemical shifts for conjugated compounds like the chromophore of a visual pigment rhodopsin (Rh). First, the calculations an applied systematically to 10 diene derivatives to obtain basic and universal relationships between their conformation and the shieldings of unsaturated carbons, It is indicated that the conjugated carbons are classified into two types according to the profiles of conformation dependence of the shieldings. The skieldings of the carbons forming the rotating bond exhibit complicated angular dependence. It is rigorously evidenced that the behavior of such carbon shieldings can be understood by considering the effect of pi-orbital modification, a new concept introduced here. On the other hand, the shieldings of the other carbons essentially follow well-known mechanisms including the steric and charge density effects. One of the most important findings is that the steric effects are reflected predominantly on the sigma(11) component, and the effects originated in electronic perturbation are on the sigma(22) and sigma(33) ones. This classification is hardly disturbed even when both types of effects simultaneously act during a conformational change, It is indicated that these basic data for the dienes are available to interpret the conformation dependence of C-13 skieldings fur more complicated compounds like retinal. Finally, combining the data for the direct ab initio shielding calculations of 11-cis-retinal and for those of the dienes, we successfully determine the preferred conformation around the C12-C13 bond of the chromophore in Rh, It is concluded that the chromophore takes s-trans conformation around the C12-C13 bond.