What I am wondering for quite some time, seeing it often when working with hair is, where these weird patterns, brightness variations along the strands (looking somewhat like interference patterns), like in the image below come from. Renderer is Cycles.
I could only figure out that it has something to do with the number of path steps (more make the patterns finer), but that doesn’t really explain it for me. Also it only happens with geometry setting ‘thick’, not with ‘ribbons’. When creating just one individual hair and looking at it up close, shading looks fine, completely smooth. In any case, there shouldn’t be any reason for number of strand segments to matter, as the hair are completely straight.
I created that example above just as a simple, minimal setup to show the problem I mean, not as something being part of an actual project. But the issue can also be seen in more realistic hair configurations. I left the hair straight to check whether it is related to large kinks in the geometry due to too little segments. IMHO it would be weird even then, as for meshes with smooth shading, no matter how low-poly they are, nothing similar happens