Numerical aperture

f-number formula

Results  f Number (f/#)  Numerical Aperture (NA) (um)  Airy Disk Diameter (um)  Gaussian Waist Size (um)  Gaussian FWHM (um)  Rayleigh Length VSWR

The f/# and NA are calculated from the aperture size (or Gaussian 1/e² beam width) and the focal length in air. Known ratios, together with the light wavelength, allow spot size calculations. This calc is accurate to ~2 significant figures.