Diffraction

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About this area

Diffraction sets a physical limit on how fine a detail a lens can form. This area explains where that limit comes from, how to calculate the size of the smallest spot a lens can produce, and how aperture choice changes image sharpness.

Start with the Airy disk calculation, then read about aperture softening and the resolution limit. Together these explain why sharpness depends on the aperture as well as on the lens design.

Going further

Why diffraction is a physical limit: diffraction is not a flaw that better manufacturing can remove. It follows from the wave nature of light passing through a finite aperture. Every lens faces it, and a perfect lens still forms an Airy pattern. The practical question is how close a real lens comes to this limit at the aperture it is used at, and how that compares with the sensor.

Choosing an aperture for a purpose: diffraction is one of several factors in choosing an aperture. Depth of field, shutter speed, noise and the lens sweet spot all play a part. A practical approach is to test the lens at the apertures you use, and use the diffraction limit as a check on what the test should show.

Articles in this area

Airy Disk Diameter Calculations: The formula, a worked example and the assumptions behind it.
Aperture-Induced Image Softening: Why stopping down trades aberrations for diffraction.
Understanding the Diffraction Limit: The resolution limit set by aperture and wavelength.

Where to go next

Return to the pillar guide to see how this area connects to the others.

How the three articles build on each other

Start with the Airy disk calculation to fix the size of the smallest spot a lens can form at a given wavelength and f-number. Use that number as a yardstick in the aperture article, where the balance between aberrations and diffraction decides the sharpest setting. Finish with the diffraction limit article, which turns the same physics into resolution and frequency terms that match the MTF charts in the lens design area.

A reader who understands the three in order can judge an aperture test result, decide whether a sharper lens is worth buying, and explain why a wide-aperture image can be soft for different reasons than a closed-down one.

Questions this area answers

Why is my image soft at f/16? Diffraction grows with the f-number, so at small apertures the Airy disk covers several pixels and fine detail blends together.

Is there a single best aperture? There is usually a middle range where aberrations and diffraction are both small. Testing at your own settings is the reliable way to find it.

What does the calculator check? It applies the Airy diameter formula to the f-number and wavelength you enter, so you can check your arithmetic and see how the spot size scales.

Limits of this area

The diffraction formulas describe an ideal, circular, unobstructed aperture at a single wavelength. Real lenses add aberrations, and real apertures may have central obstructions or non-circular blades. The results here are a baseline for judging a real lens, not a prediction of a specific product.

Working through a worked example

Take an f/11 lens at 550 nanometres. The Airy diameter is about 14.8 micrometres. On a sensor with 4 micrometre pixels, the spot covers roughly four pixels across, so diffraction is the dominant limit. Stopping down to f/16 increases the spot to about 21 micrometres, which is more than five pixels, and the softening becomes obvious in fine detail. Opening up to f/8 reduces the spot to about 10.7 micrometres, which is closer to two and a half pixels.

Who should review this: This technical documentation should be reviewed by a credentialed expert. Reviewer: [name and credential to be confirmed]. Last reviewed: [date to be added at review].

General information only. This page explains optical principles and does not replace lens design work, lens testing or advice from a qualified optical engineer.