Diffraction and the Airy disk

For an ideal circular pupil under monochromatic light, the first-zero Airy diameter on the image plane is:

λ is wavelength and N is f-number. Increasing f-number increases the diffraction diameter linearly; longer wavelengths also produce a larger diffraction pattern.

What this tells you

The result is a physical image-plane diameter. If sensor pixel pitch is known, the diameter can also be expressed in pixels for sampling analysis.

Model limits

This is an ideal circular-pupil diagnostic. Polygonal apertures, blade curvature, lens aberrations, coatings, sensor effects, and wavelength mixtures require more complete point-spread-function models.