There is a [Wikipedia article on top-left lighting](http://en.wikipedia.org/wiki/Top-left_lighting), which cites as its primary reference the papers [Where is the sun?][1] and [Is light in pictures presumed to come from the left side?][2]. These papers certainly support the conclusion that people prefer lighting from the left when _resolving a convex-concave ambiguity_.

The Sun/Perona paper notes that about 77% of paintings from a large random sampling from several museums tend towards left-lighting, which is interesting, but not a _value judgment_, and I think it is very wrong to take this kind of thing and make it a rule. Esa states the matter as a prescription: "Whenever you have a choice in it, have the light come from left side", but I think it's more likely just that "When it doesn't really matter, people creating art have a tendency to choose top-left lighting."

But maybe the old masters were on to something (and just got it wrong 23% of the time). I didn't do a comprehensive study, but a quick glance over the works of impressionists like Degas, Renoir, or Monet show that they certainly didn't hold this guideline sacred. 

And, all of that isn't photography. Edward Weston certainly never got the memo, and he's perhaps most famous for a photograph with abstract, convex shapes.

![Pepper No. 30][3]

But of course, that's just one photographer. To get a better sample, I went through Life Magazine's online collection [The Best of Life](http://life.time.com/history/the-best-of-life-37-years-in-pictures/?iid=lb-gal-viewagn#1). There, direction seems to be evenly split between a) predominantly-left light, b) predominantly-right light, c) ambiguous or mixed lighting, and c) dramatic back or front-lighting. If anything, there's a slight preference for light from the right to light from the left.

So, I put forth that while you might want to follow this suggestion in _product photography_ (and particularly when you _want to make the form of an abstract shape obvious rather than mysterious_), **there is no general rule**.


  [1]: http://www.vision.caltech.edu/SunPerona_nn0798_183-2.pdf
  [2]: http://www.ncbi.nlm.nih.gov/pubmed/15729910
  [3]: https://i.sstatic.net/VQ1gg.jpg
  [4]: https://i.sstatic.net/VYEwL.jpg