Head (and shoulders) depictions of people go far beyond the history of photography, too. Just think about the ancient Romans… Whether you really want to call them "portraits" is another thing (but then, you brought up passport pictures). Often, though, these close-up portraits were studies in preparation of the "real" work (which sometimes never happened). What the 20th century has done, is to emancipate these "fragments", "studies" etc into its own category of ("unfinished") art.
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