@David Thanks a Lot Sir . Actually I was not trying to Light the entire scene with GI alone. I've already gone through the exact setup as you've suggested. But the thing is that I need to have an independent control over the photons. So I removed the "Mia_Portal_Light" from "Photon Emitter". You were absolutely right about me not using any exposure settings for camera. Accidentally I've removed it when trying to see the GI distribution in isolation, Stupid me . Anyway here's the deal with GI :- Now when I've independent control over GI. I still cannot setup to get satisfactory result. I mean when using portal light you've to stick with "Use Light Shape" Turned On Which according to me gives a more washed out GI distribution. Maybe I might be wrong on this. But please help me out. Here's the screen shot with just the area light(on visible window)turned on with "Mia_Portal_Light" as light shader and GI. I'm having another problem with the scene file. It's having this object which disappears every time I open the scene. And only shows up when I try to manipulate it. The huge problem is that if I don't make that object appear by manipulating it. It will cause maya to crash when rendered in mental ray using my setup. Please download the file from the link below and you'll know what I mean. By the way the object name is "Wall1". The Culprit. https://www.megafileupload.com/en/file/369636/a-rar.html Thanks again.