Rayleigh Criterion (Diffraction Limit) Calculator

Any circular aperture — a camera lens, a telescope mirror, a microscope objective, even the eye's pupil — blurs a point of light into a small diffraction pattern (an Airy disk) rather than a perfect point, simply because light is a wave passing through a finite opening. The Rayleigh criterion is the standard definition of when two such points can still be told apart: they're 'just resolved' when the center of one point's Airy pattern lands on the first dark ring of the other's. That condition reduces to the closed form θ = 1.22λ ÷ D, where λ is the light's wavelength, D is the aperture's diameter, and the 1.22 factor comes from the first zero of the Bessel function describing the Airy pattern.

θ, in radians, is converted to arcseconds for readability (astronomers' conventional unit for angular resolution) and also turned into a linear distance: given how far away the target is, minimum resolvable separation = θ × distance, using the small-angle approximation that's always valid here since θ is minuscule for any real optical system.

The default 550nm wavelength is roughly the peak of human daytime visual sensitivity (green light) — a standard reference wavelength used throughout optics when no specific color is specified. Wavelength is entered in nanometers regardless of the unit toggle, the universal convention in optics worldwide; only the aperture diameter and target distance, which vary by instrument and by where you are, switch between metric and imperial.