Diffraction
The spreading of light waves as they pass the edge of an aperture, which softens the image at small f-numbers.
Formula / rule
d = 2.44 × λ × N
Measured in
µm
In depth
Diffraction is a property of light itself, not a lens fault, so no design can escape it: the smaller the opening, the more the light spreads. It sets a hard ceiling on resolution that tightens as you stop down, which is why every lens has a sweet spot — usually two to three stops from wide open — where aberrations have faded but diffraction has not yet taken over.
Examples in practice
A landscape at f/22 shows visibly less fine detail than the same scene at f/8, despite the greater depth of field.