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Why does the image of the HMS Spider look at first glance like it is scanned from an old dot-matrix type news media, with numerous small "pixels" visible, but when you click on the larger image, it is relatively sharp and clear with no pixelation visible. AnnaGoFast (talk) 23:13, 26 January 2018 (UTC)[reply]
Moire patterns. It's probably a newspaper or other halftone image originally (or may just be a scanning artefact). These have a regular single frequency component in their density. When they're resampled to downsize them, this can give rise to a beat frequency - a visible effect at a much lower frequency. Andy Dingley (talk) 23:20, 26 January 2018 (UTC)[reply]