So - having established that lights "attract" the attention of the rendering engine and help it find useful solutions to the rendering equation in fewer steps, it necessarily follows that if you have judiciously used "lights" to good effect so the rendering engine locates them with ease, then it needs fewer samples. A lot fewer.

There are different kinds of lights. It would seem, then, that there are pros and cons to these, otherwise we'd only have ONE kind of light. This brings us to another level of distinction. Point lights, spot lights, area lights, infinite lights - these all use different math with different consequences on the rendering equation. Of course since the equation is different, the samples needed becomes different.

Even if you and I use the SAME kind of light but we use different numbers of them, OR put them in different places, the resulting rendering equation is now DIFFERENT. The minimum workable samples will ALSO be different.

Trying to use the same samples (just that one number) is really setting you up for failure.

In fact, trying to use all the other numbers the same (without consideration of what they do to the rendering equation and in the context of YOUR scene) is setting you up for failure.

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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)