The first thing one encounters upon entering Joan Semmel’s Soho loft is an eerie oil self-portrait of the artist sitting on a stool, the flash of her camera exploding in a ring of light. Her silver-framed work, “Mirrored Screen” (2005), rests on a wall near the entrance of her second-floor Spring Street studio, where she has lived and worked for more than half a century. It almost looks like a mirror when Semmel stands in front of it, with her arched eyebrows and dark eyes, although her long, dark, wavy hair is now gray, streaked with white.  The piece is part of a series on locker rooms she made more than 20 years ago, when she was interested in narcissism in popular culture. Her fitness center on nearby Bleecker Street seemed like an ideal place to explore the subject as seen through other people’s bodies, so she brought a point-and-shoot film camera and asked permission to snap pictures of fellow artists and dancers while they changed.…