No lens is optically perfect. In the ideal world, every lens would produce a sharp, undistorted, evenly illuminated image that’s sharp from edge to edge. Some lenses come close, but the fact is that lens designers have to work within the limitations of the laws of physics and the glass materials they have available, and […]
Diffraction is a softening effect you see at very small lens apertures. It's caused by the way light bends when it passes a sharp edge (the edges of the aperture diaphragm blades), and it's worse at small lens apertures because a higher proportion of the light is bent compared to a relative small proportion passing unchanged through the center of the aperture.