Putting aside his self-portraits, perhaps his best treatise on the subject of portrait painting comes in his 1661-62 painting Syndics of the Drapers' Guild. Confiscated as a trademark image for Dutch Masters cigars, the work is a life-size portrait, some six feet in height and almost nine feet in width depicting six members of the guild, illuminated from the upper left by an unseen window, apparently reacting to the entrance of another individual. One is caught in the act of rising from his seat. A second figure near him reacts to this movement, while the others focus on the viewer. Each face is an individual portrait, each is lit with slight differentiation, and above all, there is a "natural" quality to the group scene that causes the viewer to consider apologizing for the intrusion.
The two elements of Rembrandt's work that make him eternal are his strong sense of composition and most importantly, his total mastery of light. His so-called "Rembrandt triangle", created by the highlighting of the cheekbone area beneath the eye opposite his light source has become stock-in-trade for portrait painters and photographers from his day to this day. Yet he used it in only one of the figures in the Drapers' Guild, demonstrating a versatility unmatched before or since in his mastery of facial lighting. In his earlier Resurrection of Christ (1635-39) we see the same mastery of light in a religious work. With this total conquest of light and shadow in mind, it's hard to imagine that during his relatively short career (a mere 40 years or so) that he could fly so high as a painter and plummet so low in popularity as to die penniless and forgotten at the age of 63.