What this tells us is that the further away the light source is from the subject and backdrop, the more likely we are to get an equal exposure from one to the other. The quality of the light source can also change with distance. You will notice that as the light gets further away it becomes a harder light with less transitional values. Also, if the subject appears to be further away from the backdrop in the last image, it's due to me needing to use a shorter focal length (zoom out) to avoid getting the softbox in the shot. The shorter focal length exaggerates perspective.
Drops & Props custom work is professional, high quality, and accurate. They were responsive to our needs and hustled to complete an order that had an impossibly tight turn around time. We ordered backdrops not only for our photo booth but to have a shared experience in every Etsy office across the globe. They made this happen for us and we are so grateful.
Again, if you are starting out or having trouble I would recommend getting the light a little further back (somewhere between 4 to 8 feet if using a similar sized light source). You will also notice that the right side of my backdrop is actually not 100 percent white, it fades to a light gray shade. I could increase my fill or correct this in post-processing, but the slight shift is intentional in this image.