I wonder if this is a bug, or if I missed something...
When a light hits tangentially convex surfaces with "smooth shading", the shadow on this surface seems to "partially ignore" the smoothness of the surface. Here is an example with a simple cylinder :
On the left, a cylinder with "flat shading". Everything's ok.
On the right, the same with "smooth". We can see an edge, bad shadowed.
With or without the "Edge split" modifier, no matters, the edge is still there. The shader is a simple white diffuse shader with no texture nor displace.
I have to add a subsurf modifier with at least 2 levels, which will insanely increase the vertex count... And I can't do that for my project.
I hope someone already saw it, and have a little tip to share. Or if someone already know it's a bug, or a limit of the current Cycles renderer ?