Airy Disc
The small bright disc surrounded by faint rings that a perfect lens produces from a point source, set by diffraction alone.
Formula / rule
d = 2.44 × λ × N
Measured in
µm
In depth
The Airy disc is the smallest spot any lens can form, so it defines the theoretical resolution limit. Its diameter grows in direct proportion to the f-number: about 5.4 µm at f/4 and 21.5 µm at f/16 for green light. When the disc grows larger than two photosites, the sensor can no longer resolve any additional detail.
Examples in practice
On a sensor with 4 µm pixels, the Airy disc exceeds the pixel size by about f/5.6, marking the onset of diffraction softening.