When photographing the Moon, a blurry image was obtained in the form of
a disk of radius r . A sharp image of the Moon would have a radius of r .
1 2
Determine how much distance you need to move the photographic plate so
that the image on it turns out sharp. The focal length of the lens is f, the
diameter is D, with r > D > r . The image area is considered to be the area
2 2 1
where light hits the photographic plate.