The Rayleigh Criterion:
The Rayleigh Criterion specifies the minimum separation between two light sources that may be resolved into distinct objects. The Rayleigh criterion is the criterion for the minimum resolvable detail.
The Rayleigh criterion is stated in the equation θ=1.22*λ/D
This equation gives the smallest possible angle θ between point sources, or the best obtainable resolution. Here λ is the wavelength of light, and D is the diameter of the lens’ aperture.
Nice website