Diffractiontechnical
Softening that appears at very small apertures (high f-numbers) on every lens.
As the aperture closes past about f/11-f/16, light bends around the blades and slightly softens the whole image. So 'more depth of field' has a limit — beyond it you trade sharpness. Stop down only as far as you need.
Use this when
- Max sharpness: stay around f/5.6-f/11
Common mistake: Shooting f/22 'to be safe' and getting a soft image.