When Nikon d700 and Nikon 70-200 lenses take photos with red background in the photo studio, what is the reason for the halo on the edge of the hair?

I dare not jump to conclusions without seeing the photos. If it belongs to the background light, it should be sought after. Whether it's a shed shoot or an outdoor backlight, it's a beautiful photo with light. When shooting in the shed, we should be good at using the hair (outline) light to make the portrait have a three-dimensional effect, but we should prevent the background light from entering the lens.

If the camera light shines into the lens, glare will appear, and sometimes even the whole photo is not clear. When shooting backlight outdoors, you should also be careful not to let sunlight enter the lens and have an angle with the light. A hood must be used.