Bill Henson used his first public speech since the furore over his photographs of naked children to explore the contradictory nature of the medium and people's reactions to it.

Speaking to a large and supportive crowd at the opening of a photographic exhibition chronicling the Asia-Pacific from the 1840s to the 1940s at the National Gallery of Australia in Canberra last night, the controversial artist said art had the power to challenge prejudices.

"The greatness of art comes from the ambiguities, which is another way of saying it stops us from knowing what to think," Henson said. "It redeems us from a world of moralism and opinionation and claptrap."

The artist ignored questions about the recent controversy over his art as he walked last night through the exhibition of more than 400 photographs.

In May the Prime Minister called Henson's depictions of naked children "absolutely revolting" and a series of police raids on art galleries hanging his work began to determine whether they were pornography. Police decided not to charge him, and the Classification Board described the pictures in question as "mild".

Henson's speech made pointed references to the art censorship row that his exhibition helped reignite. "Nothing kills the thing we love quite so perfectly as our assumption that we always know what's best, what is right for someone else, whether it's another person or another culture."

Most recently the Classification Board asked the art magazine Art Monthly Australia to submit its June edition for review. The magazine published a photograph of a naked girl, Olympia Nelson, on its cover.

"People do sometimes only see what they want to," Henson said. He recalled the comments he made when he opened an exhibition of photographs by August Sander: "Beware the loss of mid-tones."

He said: "We should be absolutely clear that we cannot know, or fully grasp, the experience that others have when they are alone - staring quietly and intently into this strange little mirror on the world - when they bring life experience to bear in each encounter with a photograph."

Photography was the most contradictory medium, he said. No other art form had the same "stupid literalness" while offering a world only reached through imagination.