Aperture Diffraction

Softness that appears when the aperture is stopped down too far. Light passing through a large opening comes out in one coherent bunch, like water from a hose at low pressure; squeeze the opening down and the light begins to scatter at the edges, just as the hose starts to spray. Past a certain aperture that scattering blurs fine detail across the whole frame no matter how good the lens is. The smaller the sensor, the sooner it arrives.