Their was one thing I wanted to do, and found out, that I am unable to. I was hoping to go into the material room, and put a transparent image file on "Main Camera" for animations. Like crosshairs and calculating (Distance) numbers, and color green (night vision).  I know most will say.."Just put a flat square in front of camera, then texture that. Yes, but if I have to move camera around for an animation scene, and that alone. I want to texture on camera, not another prop. Would be easier...wouldn't it? How can "Main Camera" be textured?