ONE OF THE MORE admirable things about Johnny Mathis is that he hasn't changed his style. While other pop performers, those with as much mileage as Mathis, have begun to sing hard, all around the note, in an effort to build a new audience, Mathis continues to sing as he did, easily, surely and with great finesse.
He always surprises. You go to one of his concerts expecting him to do nothing but the Mathis standards (''It's Not for Me to Say,'' ''Chances Are,'' ''Misty''), but there is more to the man than this. He does those, but he does others, and you may never have heard them sound any better than Mathis does them.
One is ''Vincent,'' Don McLean's song tribute to Vincent Van Gogh, which Mathis does as a salute to McLean. Another is ''Brazil,'' which is given a full, rich orchestration, and others that thoroughly please are Cole Porter's ''Begin the Beguine'' and a medley from ''West Side Story,'' which is good enough to bring Natalie Wood to mind, particularly when Mathis does ''Somewhere.''
Mathis also does a novelty song, ''Her Mother Came, Too.'' He's been doing this one for a while, and it wears very well.
Mathis doesn't do any comedy patter. He leaves that to people like Frank Sinatra, who, it is said, is the only artist to have outsold Mathis as a recording star.
All Mathis does is sing, and he does so with extraordinary artistry.
He opened the Baltimore Symphony Pops series last evening at the Joseph Meyerhoff Symphony Hall. He'll appear there tonight and tomorrow at 8:15 and on Sunday at 3 p.m. The concerts are said to be sold out, but maybe you can beg or steal a few tickets. A few seats were empty last evening.
Mathis does the second portion of the show, about one hour and 15 minutes. The BSO provides the opener, about 25 minutes of selections from Smetana's ''The Bartered Bride.''
The orchestra, of course, sounded magnificent, with and without Mathis, who also sang ''Laura,'' another pleaser. He did sing one or two other numbers that don't necessarily add to the evening, but most of his selections are winners, beginning with ''Nature Boy,'' which Mathis does as a salute to Nat King Cole.
Chosei Komatsu, associate conductor of the BSO, was conductor last evening during the opening portion of the concert. When Mathis came on, the BSO musicians were augmented by Scott Lavender, Mathis' conductor, who sat at the piano, Gil Reigers, guitar, Rich Shaw on bass and Joe Lizama, who played the drums.
A good show. No, a great show, and yes, Mathis deserves those standing ovations.