Pixel Pitch & Diffraction Limit
Pixel pitch from the sensor and resolution, the Airy disc at an aperture, and the f-number past which diffraction starts to soften detail at the pixel level.
The pixel pitch and megapixels, the Airy disc diameter at your aperture in microns and pixels, the diffraction-limited aperture by the criterion you choose, and the circle of confusion in pixels for comparison.
Example: A 45 MP full-frame sensor (8,256 px across) has 4.36 µm pixels; diffraction starts to show past about f/6.5, and at f/11 the Airy disc spans 3.4 pixels.
Smaller pixels,
earlier limit.
How pixel pitch is found, what the Airy disc is, why the limit is a convention and not a wall, and how it trades against depth of field.
Pixel pitch
Pitch is the sensor width divided by the number of pixel columns — 4.36 µm for 36 mm across 8,256 pixels. It sets the finest detail the sensor can record.
The Airy disc
A lens stopped down to f-number N cannot focus a point smaller than a disc of diameter 2.44·λ·N, where λ is the wavelength — about 13 µm at f/10 in green light. When that disc spans more than a couple of pixels, stopping down further blurs detail that the sensor could otherwise resolve.
A convention, not a wall
The limit is where the disc reaches k pixels, with k = 2 a common criterion and an input here, as is the wavelength. Past it the image softens gradually; it does not fall apart, and the extra depth of field is often worth more. At normal viewing sizes the circle of confusion, shown in pixels, is the more relevant blur.
What the number does not settle
Demosaicing, anti-aliasing filters, lens aberrations and sharpening all shift the practical limit; test your own lens. Inputs stay in your browser; the same anonymous usage counts as the rest of the site apply.