It takes a leap of faith for an artist to leave behind her songbook of hits and ask an audience to join her on a different musical journey.

But that's what Joni Mitchell did Wednesday night at the MARS Music Amphitheatre, trading her folk-pop aesthetic for a concert devoted to romantic standards. In this case, however, that leap of faith seemed more an act of self-indulgence.

It's not that Mitchell doesn't have the chops for this material. Her music has always had a jazzy sensibility. And her voice, an instrument saturated in smoke, recalls Billie Holiday in its best moments. She doesn't sing as much as she coolly wraps herself around a song -- the musical equivalent of a cat claiming your lap as a bed.

Even more to the point, Mitchell's latest album is devoted to this material -- and it's a testament to her fertile imagination. As she describes it, the recording traces "the arc of romantic love," from infatuation to consummation. The material is classic: At Last, You've Changed, Sometimes I'm Happy, Stormy Weather, etc., plus Mitchell's own A Case of You and Both Sides Now lovingly rearranged for full orchestra.

But when Mitchell sang some of these same songs -- and others -- at MARS, it didn't quite have the same effect. You could blame the orchestra, a group of local musicians -- basically, an uncredited Boca Pops -- that didn't swing with the requisite big-band flavor.

You could blame the venue: It was a crisp night made for music outdoors, but this concert seemed conceived for the indoors (and given that it didn't draw more than 5,000, why wasn't it at the Kravis Center?).

But the larger share of blame rested with Mitchell: It's OK to take chances, but the attitude that emanated on stage was one of defiance to her loyal fan base. Granted, the concert wasn't promoted as an evening of greatest hits, but even when Mitchell stayed true to her concept, she rarely seemed into it.

By contrast, when Bruce Springsteen went off on a solo acoustic tour a few years back -- a daring move for an artist who can fill stadiums -- he had a whale of a time. And when Barry Manilow played MARS just a couple of months ago, he showed how a pop star can use an orchestra to his advantage, even in an outdoor setting.

Mitchell didn't find a way to gel with her audience -- or her band. The result was a very long evening by a very great artist.