Sorry guys, was teaching at a country school today, so just got back (3 hour drive each way).

As several of you have already said, the skydome will blow out to white at these exposure levels, but so does a real photograph.And yes, the concept is to light a largish room only using external lighting with IDL.

The image I posted has no light on the floor or walls because the sun is behind the camera (over your virtual right shoulder). The image is using JUST the environmental light (from outside). I should also state that there is a window behind the camera, and the window you CAN see in the image is only half of the actual window (you can see a bit of the other half in the very bottom left of the window frame).

This isn't a 'discovery' as such; BB for one has mentioned raising light and ambient levels to compensate. My realisation was the point of comparison to roughly equate Poser light to real light and 100 ASA film. That gave me a benchmark to calculate an approximation as to what settings I'd need to get the results I'd expect. Having said that, if I were to try to emulate architectural imagery I'd have to do the same as they do; use internal lighting (mesh or otherwise) to fill the room inside, raising the amount of light hitting the virtual film. That means I don't need as long an exposure, so the external lights can be wound back to adjust. Eventually that would stop 'blowing out' the skydome.

The main issue is the artifacts along geometry seams. It looks like throwing that much light at the walls (which have genuine mesh thickness) is still letting some leak through. That's why I think having Firefly with camera type settings (focal length and apperture) would give better results. You could just set the lights to 100%, ambient to 1, and adjust from the renderer, and (hopefully) not get those artifacts. Even at 5000 samples, IC at 25% and 12 bounces I still get blotchiness. Not too difficult to correct in Photoshop, but I'd like to not have to.