I read in  that for a camera obscura the total spot diameter is equal to the diameter of the spot produced concerning geometric optics only plus the diameter of the spot concerning only diffraction. I am interested to know if this is a general result and how can it be proven?
(I would naively assume that the diameter of the spot just took the maximum of these two diameters)
Yung-Kuo, L. (editor) 2007. Problems and Solutions on Optics. New Jersey: World Scientific (p130)