Why is the second image so grainy? The light has to be found by random rays and most of those rays do not go through the windows. The only way to find the strong light is through the windows, and since most of the rays do not go there, most of the lighting is very hard to find. Therefore it takes a very large number of samples to fill in all the missing dots of light.

In the first image, the lights are inside the room and they are Poser "Lights" not geometry. Why do we even have lights? Because they're special. Lights, unlike glowing geometry, are KNOWN sources of lighting info and the render engine does not randomly shoot rays HOPING it hits a light. It specifically sends a ray DIRECTLY at the light. This is the reason we even HAVE lighting objects in renderers. If we didn't need them, we would all be using glowing polygons.

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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)