1. The problem statement, all variables and given/known data How we can determine The diameter of the lens of the camera approximitly that this picture took with that? ( we know that lens of the camera made by some parts but we count them as a thin one in Practical calculations ) 2. Relevant equations m = q / p 1/f = 1/q + 1/p f = f/p-f f : Focal length q = Distance between of the object and the lens p = Distance between of the image and the lens m = Magnification 3. The attempt at a solution i tried to find lens Magnification by calculating the distance between every centimeter of the ruler and then find its Magnification ( m ) . after that i used this formula 1/f = 1/p + 1/q so i got f Terms p now i dont know how i must find its diameter .