Airy Disk & Diffraction Limit Calculator
Use this tool to estimate the diffraction-limited Airy disk size for optics such as telescopes, camera lenses, and microscopes.
What is an Airy disk?
The Airy disk is the bright central spot produced when light passes through a circular aperture and forms a diffraction pattern. Even with perfect glass and perfect focus, a point source cannot be imaged as a perfect point. Diffraction sets a fundamental limit on sharpness. That limit is often called the diffraction limit.
In practice, this matters for astrophotography, planetary imaging, microscopy, and everyday photography at small apertures (high f-numbers). If your optical system is diffraction-limited, the Airy disk size determines the best possible detail you can resolve.
Formulas used in this calculator
1) Angular radius to first minimum
Where:
- θ = angular radius (radians) to the first dark ring
- λ = wavelength of light
- D = aperture diameter
2) Airy disk diameter on the focal plane
Where N is the f-number. This gives the physical spot diameter at the image plane (sensor/film) when focus is ideal.
How to use the calculator
- Enter your wavelength (550 nm is a good default for visible light).
- Enter aperture diameter in mm.
- Enter focal length in mm to get the focal-plane Airy size.
- Optionally add pixel size to see spot diameter in pixels.
- Click Calculate.
Example interpretation
Suppose you use a 100 mm aperture at 1000 mm focal length and 550 nm light. The calculator will show:
- Angular diffraction limit in arcseconds (how finely the system can separate distant details).
- Airy disk diameter in micrometers on your sensor.
- Equivalent f-number and optional pixel sampling estimate.
If the Airy diameter is much larger than a pixel, your system may be oversampled. If it is much smaller than two pixels, you may be undersampled for fine-detail work.
Why this matters in photography and astronomy
Photography
Stopping down (higher f-number) improves aberrations but increases diffraction blur. There is always a trade-off: at some point, stopping down further makes images softer despite better depth of field.
Astronomy
For telescopes, larger aperture reduces diffraction blur and improves theoretical resolution. Real-world seeing and tracking may dominate, but diffraction still defines the optical floor.
Microscopy
In microscopes, diffraction and numerical aperture jointly set the smallest resolvable features. The same physics applies: wave optics creates a finite point spread function.
Practical tips
- Use realistic wavelengths: red (~650 nm), green (~550 nm), blue (~450 nm).
- For broadband imaging, green is a common reference estimate.
- Remember atmospheric seeing can be worse than diffraction for telescopes.
- This model assumes an ideal circular aperture and does not include central obstruction, aberrations, or turbulence.
FAQ
Does this include optical defects?
No. It computes the diffraction-limited ideal case only.
Is this the same as Dawes limit?
Not exactly. Dawes is an empirical rule for double-star separation; this calculator uses wave-optics diffraction equations. Both are useful reference points.
Can I use it for camera lenses?
Yes. Enter aperture diameter and focal length (or infer one from the other using f-number relationships) to estimate diffraction-limited spot size.
Bottom line
The Airy disk is the unavoidable sharpness boundary created by light itself. Use this calculator to understand your system’s theoretical limit, compare setups, and make informed choices about aperture, focal length, and sensor sampling.