Jasper Johns is a famously reticent artist. So it may strike some visitors to the Museum of Modern Art as comical when they read his words quoted in a wall label next to his 1985 painting ''Summer'': ''In my early work I tried to hide my personality, my psychological state, my emotions, but eventually it seemed like a losing battle. Finally one must simply drop the reserve.'' Funny that Mr. Johns would view his later work, which is not much less enigmatic than his early work, as unreserved. The quotation appears in ''Focus: Jasper Johns,'' an exhibition of 87 paintings, drawings and prints from the permanent collection organized by Deborah Wye, the museum's chief curator of prints and illustrated books. The show focuses on a handful of motifs that Mr. Johns has recycled over the years, including the target, the numbers, the flag, the can of brushes and the so-called crosshatchings. Ranging in time from his first painting of the American flag in 1954-55 to a recently acquired series of works on paper from 2001, it would make an excellent introduction for anyone just getting to know Mr. Johns's work. [ABSTRACT FROM PUBLISHER]