the second thing was: does it help to change the direct point light to an object with high ambience only.

No, it does not. The attached does such a thing, ambient full white with vaue 400 to get a comparable lighting level in the room, and splotches all over the place: see the image - made at HIGH QUALITY render settings!!

Even when all four quality parameters are set to the max (as in the standard FireFly render settings window): splotches.

Hence the basic lesson is: when IDL rendering, hotspots on objects by extreme lighting level should be avoided, even when these are out of the camera view. Whether the extremes are caused by inverse quare direct pointlights or by hi-ambient IDL sources. Whether you drill holes in the ceiling out of the camera view, use light-absorbent materials on the walls or apply spcific attenuation profiles for the direct lights themselves: as long as the hotspots are avoided you're fine. And... when direct pointlights are used, really high quality settings are a proper way out when the other options fall short or are unapplicable for whatever reason.

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Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though