“My work is purely autobiographical,” Lucian Freud once said. The British painter, known as one of the foremost 20th-century portraitists, was referring to his work as a whole. But if you think about it, it’s a wonder that an exhibition of Freud’s self-portraits had not been presented until last October, at London’s Royal Academy of Art. He’s one of the few 20th-century artists who consistently turned his brush to his own image throughout a long career. “Lucian Freud: The Self-Portraits,” which gathers nearly 50 autobiographical works to trace the arc of the painter’s life and work, comes to the MFA, Boston, after it closes in London on January 26. —J.V.