It's hard to imagine the great cinema of the '60s--or any great cinema, for that matter--without the work of Nykvist; he's practically defined the look of the period for many of us, particularly the kind of spiritually austere, introspective psychodrama he and filmmaker Ingmar Bergman were working on at this time. From the "Trilogy of Faith" (Through a Glass Darkly, Winter Light, and The Silence), to the experimental Persona, to the horror-film Hour of the Wolf to the great Shame (my favorite of his films of this period), he's lit many a classic with that unique frosted-glass look of his, as if sunlight was cold instead of warm; as if it were filtered through a chilled windowpane, darkly.
Maybe the single most amazing trait of Nykvist's was that he achieved many of those magical effects through mostly simple techniques; as Bergman once said of the man in Bergman on Bergman: "All he needs to work is three lamps and a little greaseproof paper." He could in effect sweep aside all the technical paraphernalia associated with his trade, home in on the dramatic essence of the scene, and capture it on film for his director.
He worked for other directors, most notably Roman Polanski (The Tenant), Woody Allen (Crimes and Misdemeanors), Philip Kaufman (The Unbearable Lightness of Being); his finest non-Bergman work is arguably Andrei Tarkovsky's The Sacrifice, with its final six-minute shot (the first take of which had famously jammed, requiring a very expensive reconstruction and reshoot). If Nykvist had been known only for these films, he'd probably be considered a great cinematographer; as is, he is legend. The world is all the dimmer without him.