Check your D-Lighting settings on the D610. Just because you shot raw doesn't mean Capture or Lightroom shows you an untouched file. AFAIK it still applies camera settings for the preview. If D-Lighting was set to normal or high, you can't really judge the accuracy of the shadow exposure. In addition, since you're using a DSLR to 'meter', you should really be setting the proper exposure on the subject first (either with BG lights off, or BG cropped out of the frame), then adjusting the background lights to suit. Best solution is an inexpensive handheld flash meter. You will have consistency between mediums without regard to a digital camera's settings. Others have expressed an problem with the scans as well, and I agree. There is still a long way to go to blow out the background on the first shot and the scanner tried to compensate for it. The last frame has more of the subject in the frame, therefore the scanner was able to set exposure correctly and, as you can see, the background looks just right.