Was Descending Staircase serious, a spoof or both? What did Duchamp bring to Cubism that had not previously been seen

The answer is both. Despite most of Duchamp's work was a sort of spoof (including "N*ude Descending a Staircase"), the consequences of his trajectory represent a critical watershed in the History of Art.

changeable lenses of different focal lengths allows light to enter the camera,which then forms the image on light-sensitive film;