Path tracers are known for certain limitations. Chief among them is that they have trouble consistently locating paths that reach small sources of intense light.

I ran this render for eight hours and it's a dismal failure. Results like this are why people give advice to use area lights. But - while that is a solution for some situations, it's not a good solution. In this case the light bulb is not at all going to be simulated well by a point light. The advice to use area lights is misguided - it is given and repeated because A) certain people who know some things don't know everything they should about the tools, yet partial knowledge gives them some credibility and B) others who know even less just believe anything said but somebody with more knowledge.

Anyway - here's what you get if you lack material skills.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)