My enlarger came in the mail today and I have two questions about it. Under the enlarger head, by the lens, there is a knob that turns a red disc over the lens inside the inlarger head. What is the red light used for? Also, in my 35mm film carrier, there are two red rectangles. One is above the negitive and the other is below. They are about 1/3 the length of the exposure. Any idea on what those are for?