I don't do photography but I found an interesting phenomenon when taking pictures of some docs. The camera is a smartphone and there was 1 long (I think incandescent) light above. When I try to take a photo of some paper, the phone itself kinda blocks the main light, casting a shadow so I got the idea to use my small flashlight with a long light along its body for better lighting. However, I saw that when both the main & flashlight lights were on, there'd be something like `| | | | |` but turned 90 degrees (equal & parallel bars of dark & light) scrolling along vertically in the phone scree…