Diffraction is a physical property of all waveforms, including light. In lenses, diffraction is most commonly associated with a lowering of sharpness and contrast at smallest apertures (biggest f-numbers), and while that indeed is the most blatant affect of diffraction, the entire picture (pun intended) is more complicated. If you can cope with some theory and oft-unmentioned aspects, see the separate JAPB article on diffraction here.

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