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.

Input

Focal length (f)?

Entrance pupil diameter (D)?

Medium refractive index (n)?

Wavelength (λ)?

F-Number & NA

f-number (f/#)

2.0000

Cone half-angle (θ)

14.036°

NA (exact)

NA = n × sin(arctan(D/2f)) = 1.0000 × sin(14.036°)

0.24254

NA (paraxial approx)

NA ≈ n / (2 × f/#)

0.25000

Diffraction Limit

Airy disk radius

r = 1.22 × λ × f/#

1.3420μm

Étendue

Étendue (G)

G = π × n² × A × sin²θ (based on entrance pupil)

90.713mm²·sr

Abridged Optics — F-Number & NA Calculator v1.0The exact NA formula accounts for the nonlinear relationship between cone angle and f-number. The paraxial approximation is shown for comparison and is accurate for f/# > 2.