F-Number & NA Calculator
Calculate f-number, numerical aperture, cone half-angle, Airy disk radius, and étendue. Convert between f/# and NA for lenses, objectives, and fiber optics.
F-number (f/#) and numerical aperture (NA) are two ways of expressing the light-gathering angle of an optical system. F-number is defined as focal length divided by entrance pupil diameter; NA is n·sin(θ) where θ is the marginal ray half-angle and n is the refractive index of the image space. They are related by f/# ≈ 1/(2·NA) in air. This calculator converts between f/# and NA, computes the cone half-angle, estimates the Airy disk radius (diffraction limit at a given wavelength), and calculates étendue — the product of area and solid angle that is conserved through a lossless optical system. Use this tool when matching lens apertures to fiber NA, sizing aperture stops, or estimating diffraction-limited resolution.