Jennifer Lopez is not just famous for her music and movies, but also her voluptuous booty ... just ask the guy who fell under its trance at the Sundance Film Festival.

J Lo looked amazing in her black lace gown as she strolled into the Sunday night premiere of her film, "Kiss of the Spider Woman" in Salt Lake City, Utah -- and, of course, she was the center of everyone's attention, and one fella in particular.

Check out this photo ... which shows the man standing behind J Lo with his eyes laser locked on her shapely butt. The dude definitely had the best view of anyone in the place -- and J Lo seemed oblivious to his attention.

As you know, J Lo attended the screening with director Bill Condon, costar Tonatiuh and others. She posed for paparazzi photos on the red carpet before heading into the theater to watch the film, which was produced by J Lo, her ex-husband Ben Affleck and Matt Damon.

The movie is about a gay hairdresser, Luis Molina, who's imprisoned for 8 years for corrupting a minor in 1980s Argentina. Molina creates a fantasy woman in his mind named Aurora, played by J Lo, to escape the horrors of prison.