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Sounds very much like a Fresnel Zone Plate effect. Search for that on Google and you'll find all you need!
P.S. A pinhole camera focuses the image by being, essentially, the central circle of a Fresnel zone plate.
P.P.S. It is diffraction. What do you think makes shadows 'blurry' the further they are from the object blocking the light? That's diffraction too!
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