What is a diffraction-limited aperture, and when does it start to matter?
A lens becomes diffraction-limited once the blur disc that diffraction itself produces (the airy disk) grows larger than your sensor's circle of confusion — using green light as a reference wavelength, that crossover lands around f/21 on full frame (0.0288mm circle of confusion) and around f/14 on APS-C (0.0188mm circle of confusion), meaningfully earlier on the smaller, stricter-threshold sensor.
For the underlying optics — what an airy disk actually is and why narrowing the aperture makes it grow — see the diffraction glossary entry. The short version: it's a fixed physical spreading effect that every lens produces, and "diffraction-limited" just marks the aperture where that spread outgrows the sensor's own circle-of-confusion threshold and becomes the dominant thing softening the image.
Why the Crossover Point Depends on Sensor Size
Diffraction's airy disk size depends only on wavelength and f-number — it doesn't know or care what sensor is behind the lens. What changes by sensor format is the threshold it's compared against: a smaller sensor's stricter circle of confusion means diffraction becomes visible at a wider aperture (a lower f-number) than it would on a larger sensor, which is exactly why APS-C and Micro Four Thirds bodies reach their diffraction-limited crossover sooner — at a less narrow aperture — than full frame does.
Where the Crossover Lands, Format by Format
Run the same 2.44-times-wavelength-times-f-number formula through Micro Four Thirds' own circle of confusion (roughly 0.0144mm) and the crossover lands around f/11 — noticeably earlier than APS-C's f/14, which is itself earlier than full frame's f/21. Medium-format digital runs the pattern in reverse: a larger, more forgiving circle of confusion pushes the crossover out to around f/27 on a typical 44x33mm sensor. None of this means a lens is unusable past its format's crossover — it means diffraction has taken over from focus accuracy and lens aberrations as the main thing limiting resolution at that aperture, which for a landscape frame that needs the depth of field a narrow aperture buys is very often still the right tradeoff to make.
See where diffraction and depth of field trade off for your own sensor and aperture in the DoF & Hyperfocal Calculator, pre-filled near full frame's diffraction crossover.
Frequently Asked Questions
Does diffraction-limited mean my photos become unusable past that aperture?
No — it means diffraction becomes the dominant softening factor rather than an invisible one, and the effect grows gradually, not suddenly. Many photographers shoot a stop or two past the theoretical crossover point when the depth-of-field benefit is worth the modest softness cost.
Why do higher-resolution sensors seem more sensitive to diffraction?
Higher resolution doesn't change the physical diffraction effect itself, but it does let you view images at a scale where the airy disk's size relative to a single pixel becomes more visible — the diffraction is identical, the scrutiny it's viewed under is what changed.