Hey everybody, I'm right now trying to solve a question I found in a German physics competition 2006 concerning the diameter of a thin camera lens. I'm not able to find a solution, since there is basically nothing given. Here the task (I underlined phrases which might be important): A ruler was photographed. There was a chain of lights with small lamps paced far behind the ruler, so that the lamp occur to unclear. You are allowed to assume, that the picture was taken by a single, very thin lens. Find the diameter of the lens by using the photo. The only given value is the diameter of the unclear circles caused by the small lamps (by using the ruler on the photo). I know some laws and how to make these "ray diagrams" with lines, but I have no Idea how to solve this task and what is needed, especially since I have never seen a formula (or so) that includes the diameter. I hope you can help me finding a solution.