In New York, between 1953 and 1961, Robert Rauschenberg and Jasper Johns were in a clandestine relationship at a time when American laws made same-sex relationships illegal. It was a time when angst-ridden Abstract Expressionism was in vogue and the art of Rauschenberg and Johns with its enigmatic everyday found objects appeared as strange and subversive.

Almost a decade after they parted company, their paths once again crossed. In the late 1960s both found themselves working with a printer of genius, Ken Tyler, who as a dedicated midwife set out to transfer their somewhat wacky ideas of visual codes into lithography and screenprints.

James Mollison, the visionary founding director of the National Gallery of Australia, acquired much of the early Tyler print archive and subsequently the print curators Pat Gilmour and Jane Kinsman built on these foundations to create the pre-eminent Tyler collection of prints and documentation in Canberra. These are the circumstances behind this exhibition.

The fact that the show draws so heavily on the Tyler collection does, almost through necessity, mean that this is neither a coherent nor comprehensive exhibition but focuses on some of the individual highlights of these artists' print oeuvres. For example, we see none of the Rauschenberg prints for Tatyana Grosman, a key figure among the nearly 1000 prints to his name. Many of the prints on display have been shown at the gallery before. Some have not been seen for quite some time and do present milestones in the history of American printmaking.

Rauschenberg's life-size self-portrait titled Booster, 1967, was the largest hand-pulled lithograph made up to that date. In part it was inspired by Rauschenberg's fascination with space exploration and the idea that his body could be conceived as an elaborate machine for movement. Tyler first made a series of composite X-rays of Rauschenberg's nude body (except for his boots) and then transferred the negatives photographically onto lithographic plates. This almost nude image is surrounded with magazine illustrations, including several from a 1962 Life article on the Mercury space program, together with some objects from the artist's surrounding world, creating a somewhat nonsensical visual code. Subverting the harmony of the lithograph is a celestial chart for the year 1967 that has been screenprinted over the entire image.

Booster is a work of enormous presence and in retrospect has served as a benchmark against which further developments in lithography were placed concerning the scale and potential for printmaking. I recall Tyler once telling me that Rauschenberg, on seeing the early X-ray prints, was amazed that he appeared as such a "well-hung" individual.

Johns' lithographs, Color numeral series, 1968-69, and Black numeral series, 1968, are the showstoppers in this exhibition. Again, his early prints were with Tatyana Grosman and among the more than 400 prints that he has made to date, the Target and Ale Cans series have become some of the most iconic in American art. The numerals appear to elevate the banal and obvious to the status of iconic and enigmatic. In a 1960s notebook, Johns states, "Take an object. Do something to it. Do something else to it."

Technically, the numerals lithographs are immensely challenging - it is impossible to see how they were done - but somehow Tyler met the artist's challenge and created this beautiful and mysterious wall of obvious but at the same time enigmatic images.

With so much of the National Gallery closed, this exhibition is a little gem that is full of surprises.