Portraits feel most natural to our eye when the light falls in from the left in a 45° angle, imitating the sun.
In the book I am reading from 1878 they say roughly the same thing. They didn’t have lamps yet and taped the windows until they had the desired light incidence. There are four examples of where the light comes from (side, up, below, front) and the conclusion is that from upper left it looks most real. It is the only photo in the book, it’s glued in there and mentioned as a great ‘feature’ in the foreword.
Vice versa having light from underneath looks unatural and reveals the use of artificial lighting.
In this practice during the light module we played around with backlight.