I don’t know if this is the right place, but there are probably people here with a lot of experience that may answer this.

I’m trying to model a laser system, where I want to know the focal spot size of the laser.

For this there is a standard, generally used formula: (M² * focal length * wavelength) / (pi * aperture). For parameters 980nm wavelength, 330mm focal length and aperture 20mm, this gives 20,6µm.

For the Airy disk radius, the formula is (1.22 * wavelength * F/#). For the same parameters, this gives 19,73µm, which is also what I get from Zemax. This means the Airy diameter is 39,46µm, which is about twice the size of the focal spot size calculated with the generally used formula to calculate the focal spot size.

So I am wondering what is the difference here?