Lens diffraction is the softening of an image that happens when light bends slightly as it passes the edge of a small opening, in this case your camera’s aperture. It is a property of light itself, not a lens flaw, and every lens on every camera is affected by it. The key fact to understand, and the one most explanations of this topic get backward, is which direction it runs: diffraction gets worse as the aperture opening gets smaller, meaning it gets worse at higher f-numbers like f/16 or f/22, not at wide apertures like f/2.8.
Why a Smaller Opening Spreads Light Out More
When light passes through an opening, it does not travel through in a perfectly straight bundle of rays. It bends slightly around the edges of the opening, and that bending spreads a single point of light into a small blurred disk on the sensor called an Airy disk, after the astronomer who first described it. A wide aperture has a large opening relative to the amount of bending at its edges, so the Airy disk stays small and the point of light still looks essentially like a point. A narrow aperture has a small opening, so the edge-bending affects a much larger share of the light passing through, and the Airy disk grows.
The size of the Airy disk follows a simple relationship: its diameter is roughly 2.44 times the wavelength of light times your f-number. Wavelength does not change, so the only variable you control is the f-number, and the disk grows in direct proportion to it. Stop down from f/8 to f/16 and the Airy disk roughly doubles in diameter. This is the opposite of what the phrase “stopping down” suggests to a lot of photographers, who associate a higher f-number with a crisper, more corrected image because stopping down does reduce lens aberrations like coma and field curvature. Both things are true at once: stopping down fixes some problems and creates a different one, and the two effects move in opposite directions as you close the aperture.
When Diffraction Actually Becomes Visible
An Airy disk exists at every aperture, including wide open, so diffraction is technically always present. What changes is whether it is large enough to matter next to your sensor’s pixel pitch, the physical width of one pixel. When the Airy disk is smaller than a pixel, it has no visible effect: the pixel cannot resolve detail finer than itself anyway. Once the Airy disk grows larger than a pixel, it starts to spread light across neighboring pixels, and the image begins to soften at the pixel level.
This is why the same f-number behaves differently on different cameras. A sensor with larger pixels, whether because it is a larger sensor or because it has fewer megapixels for its size, can tolerate a smaller aperture before diffraction becomes visible at 100% zoom. A sensor with smaller pixels, whether because it is a smaller sensor (APS-C, Micro Four Thirds, a phone) or because it is a very high-resolution full-frame body, starts showing the effect at a wider aperture. A rough way to estimate your own camera’s diffraction-limited aperture is to take your pixel pitch in micrometers and divide by about 1.3; the result is roughly the f-number where diffraction begins to cost you pixel-level detail. Full-frame cameras with moderate resolution typically reach this point somewhere around f/11 to f/16. Smaller sensors and very high-megapixel bodies often reach it by f/8 to f/11.
How much this matters to you also depends on how the image is viewed. Diffraction that is visible at 100% on a screen can disappear entirely in a normally sized print or a web-sized image, the same way high-ISO noise or minor focus error can. If you always crop in tight or print very large, take the diffraction-limited aperture seriously. If your images are mostly viewed at normal sizes, you have more room to stop down before it matters.
Finding Your Sweet Spot
Most lenses are not at their sharpest wide open either, because wide apertures bring out their own optical aberrations. The result is a sweet spot somewhere in the middle of the aperture range, where aberrations have faded but diffraction has not yet taken over. For most full-frame cameras this lands around f/5.6 to f/11. Cameras with smaller sensors or higher pixel density tend to have their sweet spot shifted a stop or two wider, closer to f/4 to f/8, because they hit the diffraction-limited aperture sooner. The only way to know your own lens and camera combination precisely is to shoot a test scene with fine detail at every full stop and compare the results at the magnification you actually use.
The Trade-Off Against Depth of Field
Diffraction puts photographers who need a lot of depth of field, landscape and macro photographers especially, in a genuine bind. Extending depth of field means stopping down to a smaller aperture, but stopping down too far trades sharpness in the focal plane for diffraction softening across the whole frame. A landscape shot at f/22 for maximum depth of field may actually look less sharp overall than the same shot at f/11, even though less of the scene is in focus at f/11, because the plane that is in focus is meaningfully crisper.
The practical answer is to stop down only as far as the scene actually requires. Use a hyperfocal distance calculation or a depth of field calculator to find the widest aperture that still covers the depth you need, rather than defaulting to the smallest aperture available. Try the challenge below to see how aperture, depth of field, and diffraction interact in a real scene.
When a single aperture cannot deliver both the depth of field and the sharpness you want, focus stacking avoids the trade-off entirely. Shoot a series of frames at a wider, less diffraction-affected aperture, each focused at a different distance through the scene, then blend the sharpest parts of each frame together in post-processing. This lets you achieve front-to-back sharpness that would require an aperture so small that diffraction alone would ruin it if you tried to get there in one exposure.
Further Viewing
Here are some videos about this topic.
A great explainer here from Don Komarechka:
In this video by Tony Northrup gives a great walkthrough on this topic and the science behind it:
Steve Perry outlines everything you need to know about lens diffraction:
The video is loaded with practical examples that answer questions like:
- How far can you safely stop down?
- Does diffraction limit the usefulness of high res sensors?
- Can you sharpen out diffraction?
- Should you ever use small F/stops?
- What causes diffraction in the first place?
- What’s the difference between sharpness and depth of field?