So you can add each light individually and recreate what they look like when they’re all on. But what if you’ve got light pollution? Originally I did this with a closed bathroom door and a towel along the door seam. What if you’ve got a bathroom window, or an open door?
I took a starting shot with no lights on, but WITH hallway pollution. Then, one shot per six light sources (3 bulbs, a bedlamp, my cell phone flashlight, and another flashlight). Each of those shots also had the light pollution.
But hey! Light pollution + a light source is still linear addition. So with that light pollution establishing shot, you can use linear subtraction.
First I add a layer to the hallLights. Then I subtract a duplicate of the hallLights layer. Then add another layer, and subtract again. Add subtract add subtract. The final result is identical to the photo where all 6 lights were on! (7 lights, if you count the light pollution.)
The Og Bog Blog 