Diffraction is discussed in detail in Making Sharp Images.
Stopping down too far begins to degrade contrast at first, then starts destroying resolution.
Diffraction doesn’t care which camera is used or its resolution, but a high-resolution sensor suffers more in terms of per-pixel quality, because diffraction blurs the resolved spot size more relative to the size of the sensor photosites, “spilling over” a single photosite, reducing the contrast and then the resolved detail.
With the Nikon D800, diffraction can be noticed beginning at ƒ/5.6, It steadily increases in negative effect at smaller apertures (ƒ/8, ƒ/11, ƒ/16, ƒ/22).
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