Airy Disk Diameter Calculator

Enter your optical parameters, choose an output mode, then submit. The result will appear above this form.

Output mode

Use angular for telescopes; use image-plane for sensors.

Wavelength (λ)

Visible light is often 450–650 nm.

Medium refractive index (n)

Use 1.000 for air. Increase n for immersion media.

Aperture diameter (D)

Required for angular results and for f/D.

Focal length (f) (optional)

Used to compute N = f/D if needed.

f-number (N) (optional)

If left blank, N is computed from f and D.

Reset

Formula used

The Airy pattern describes the diffraction spot formed by a circular aperture. The first dark ring occurs at a radius set by wavelength and numerical aperture. In the focal (image) plane, the spot size scales with f-number, while in angle space it scales inversely with aperture diameter.

Effective wavelength: λ_eff = λ / n
Image-plane diameter: d = 2.44 · λ_eff · N
Angular diameter: θ = 2.44 · λ_eff / D

These relations are commonly used for diffraction-limited lenses, telescopes, and microscopes with circular pupils.

How to use this calculator

Select an output mode: image-plane, angular, or both.
Enter wavelength and choose its unit.
Set the medium index n (use 1.000 for air).
For angular results, provide the aperture diameter D.
For image-plane results, provide f-number N, or provide f and D.
Click Calculate to show results above the form.
Use the CSV or PDF buttons to export the result block.

Example data table

Wavelength (nm)	Medium n	f-number (N)	Diameter (μm)	Aperture (mm)	Angular diameter (arcsec)
550	1.000	4	5.37	50	5.53
532	1.000	8	10.38	25	10.70
850	1.000	2.8	5.81	100	4.28
550	1.330	4	4.04	50	4.16

Example values assume circular apertures and diffraction-limited conditions. Real systems may be larger due to aberrations, defocus, or atmospheric seeing.