I have been messing around with trying to optimize the lighting for an interior scene and thought I would share what I have found to serve as a guide. Hopefully this will help others with the constant trial and error. The trick is to balance the portal lights so that they affect the scene while adding light in specific areas.
My scene is a condo in Grand Cayman with big windows behind the camera and a hallway with a glass door at the other end. I created a plane light for each window and placed it inside the room just inside the glass. Make them portal lights in the parameters and set them to 100 intensity. Be sure to turn off Affect Reflection if you don't want them to reflect off the shiny surfaces in your room. It is not necessary to turn on Invisible as that doesn't seem to affect the plane since they are portals and that is redundant.
Now comes the interesting part. You could just adjust the camera to adapt to the portal lights but my clients often want more control over how the areas of the room are illuminated. So I found the best method is to use mesh lights to flood certain areas of the room with additional light. Mesh lights are still a work in progress at this point so be careful. I create a rectangular box about 24 inches high that is about 50% of the size of the area I want to illuminate. Place it a little lower than mid way between the floor and ceiling and make sure it doesn't touch anything. If you get your mesh light too close to the ceiling you will get strange light patterns from the box shape. Make them invisible and set their intensity to 25%. IMPORTANT: Do not rotate these mesh lights! Currently that corrupts them. I also ran into issues when scaling their size so create a new light for each area. Hopefully the next release will fix a few of these quirks since Tech is aware of them.
Now you can go to your camera settings. For this room and my conditions I set my ISO to 300, aperture of 11 and speed at 120. This is close to real world photography settings so if you are familiar with cameras that helps.
Other things of interest: If you have very reflective surfaces (in my case high gloss cabinets) then you need to place a plane with the reflection texture map behind the camera. Turn off shadow casting and receiving for the plane and set it to Emissive in V-ray materials. My scene was the sky and ocean behind me which added color to the reflective cabinets and atmosphere to the room. These are apparent in the Raw Reflection channel which I overlay in PS using the soft light layer mode. I use the Reflection Channel over that and set it to Screen mode. Adjust the levels as needed. Always include a Material ID output channel in your render so that you can adjust all your materials later.
Now there has been a lot written about Noise in the reflection channel in all V-ray platforms and now I will address it for Form.z. As the distance from the camera increased in my scene the noise got much worse. My past solution was to Despeckle my Raw Reflection Channel in PS but that often led to a muted and blurry reflection. Tech informed me that I need to add more light to that area to supply more rays or I needed to render out at a much higher resolution (6000 px wide). Adding light might not be what you want and huge renders take time. There is another option which is under V-ray settings- Raytrace- Optimization - Shading rate. Default is 8 and you can set that to 16 to 32 which slowed the render but cleaned up the noise quite a bit. While you are in the settings you can also solve another issue if you have multiple transparent surfaces overlapping each other ( a glass bowl on a glass table in front of a window for instance). Your alpha channel will not render correctly unless you increase your Max Trace Depth. Default is 5 but you might need to increase it to 6 to 8.
There are a lot of other little tweaks at our disposal but this seems to be a good starting point for my images. I would be very interested to see how others are setting up their lighting for interiors and how it affects quality and render times.
PS. I tried to post an image but this forum is still broken.