The photo cell watches the light and if it changes it tells the timer and the time will lengthen or shorten depending on the out put of the light. this way print number one matches print number 20. I live in an area where the power would change while I'M printing and this causes the out put of the light to change. Once I started testing the timer my prints were consistence. I also did the Fine Prints for Zone VI and would print a hundred prints at a time. The Cold light would change the out put as it heat up and cool down. Before the timer the prints would vary. After the timer the print were all alike.