Optical to infrared conversion using galaxies in the original S⁴G sample. The left panel shows the i − 3.6 μm colour versus the stellar mass relationship. The right panel shows the i − 3.6 μm colour plotted against the CVRHS revised T −types. To have a more homogeneous distribution in the right-hand panel, we added a random offset according to the uncertainties of the morphological revised T-types as reported by Buta et al. (2015), from a normal distribution with a width of their uncertainties, N(0, 0.52).