Diffraction
Diffraction is light bending slightly around the edges of the aperture opening, spreading a theoretically perfect point of light into a small disc (the airy disk) even at flawless focus. A wider aperture keeps this spread small enough to be invisible against everything else limiting sharpness; a narrower aperture makes the spread larger, and past a certain point it becomes the dominant source of softness across the whole frame.
The aperture where diffraction starts visibly limiting sharpness depends on sensor size, since it's compared against each format's own circle of confusion — roughly f/21 on full frame and roughly f/14 on APS-C under this site's diagonal/1500 convention, meaningfully earlier (a wider aperture) on the smaller, stricter-threshold sensor.
Frequently Asked Questions
Should I avoid narrow apertures entirely because of diffraction?
No — diffraction's softening effect is gradual, not a sudden cutoff, and the depth-of-field benefit of a narrow aperture is often worth a modest, barely visible diffraction cost, especially for landscape work needing maximum front-to-back sharpness.
Does diffraction affect every part of the frame equally?
Yes — unlike lens aberrations, which often vary between center and edges, diffraction is a uniform effect across the entire frame at a given aperture, since it depends only on the aperture opening itself, not on position within the image circle.