EXCERPT page containing first few paragraphs. 2023-03-29 18:59:03
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Influence of Pixel Size
When rays of light pass through an opening (e.g., a lens diaphragm), they form a wavefront interference pattern know as the Airy disc (Huygens-Fresnel principle).
The Airy pattern with all of its rings has an identical shape for any perfect lens with a round aperture. The only thing that changes with stopping down is the size of the Airy pattern, scaling linearly with the f-stop. This assumes a perfect aberration-free lens.
As the hole (lens diaphragm) becomes smaller (“stopping down”), the scattering effect increases; this causes a drop in contrast. At the same time, the Airy disc steadily becomes larger, with its diameter placing a fixed limit on resolution at any given aperture.
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Note how the Airy disc begins to cover more than one photosite at apertures f/8 and smaller.
The diffraction limit is reached when two Airy discs overlap by half their diameter.