Why can't more megapixels beat the diffraction limit?

A lens cannot focus light to a perfect point, however well it is made. Light is a wave, and when it squeezes through a round opening it spreads out. The smallest spot a perfect lens can form is called the Airy disk, the best-focused spot of light that a perfect lens with a circular aperture can make, limited by the diffraction of light. This sets a limit to the resolution of any image, however perfect the lens, and a system limited only by diffraction is said to be diffraction limited.

How big is the spot? It is characterized by the wavelength of the light and the size of the aperture. The smaller the opening, the more the light spreads: as you decrease the aperture, diffraction proportionately increases, and at small apertures such as f/22 most modern lenses are limited only by diffraction.

That is a trap for photographers. A smaller aperture gives a longer depth of field, so stopping down seems like the way to a sharper landscape. But diffraction grows too. For an f/8 lens and green light, the spot is about 9.76 micrometres across, similar to the pixel size of most full-frame cameras. So at f/8 the sensor's pixels and diffraction already both limit sharpness, and stopping down further makes the blur spot bigger than the pixels, so extra pixels only record the blur in more detail.

The limit was first seen in the sky: John Herschel described a bright star surrounded by rings in an 1828 article, and George Airy gave the first full theory in 1835.