Enter either the f-number in air (f/#) or the numerical aperture (NA) of the lens, along with the light wavelength of interest. The calculation will find the airy disk size (assuing a homogeneous beam) or the Gaussian waist (1/e²) or FWHM diameter. The Gaussian calculation assumes an untruncated beam with the NA applying to the (1/e²) diameter. For a truncated beam at the same width, multiply by 1.12. The Rayleigh length is calculated in air.