This example's from A Bridge Too Far, though one can see it in other films from the decade. (Klute has my favorite use of it.) I could guess some reasons for this stylistic tic, but truth is, I don't really know what drove the fashion for it. Certainly technological changes in cinematography proceeded rapidly in the decade, but the cinematography of the films in which flare appears is otherwise so controlled and crisp, it's hard for me to imagine it being anything but intentional, or at least built into a conscious aesthetic.
Cinematic storytelling: A podcast on narrative
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