Hockney's swimming pool photographs are well-known

And next week Thames and Hudson will publish his book further exploring the idea that western art has been dependent on optical devices for some 400 years.

"We thought we saw the 20th Century on the news and elsewhere better than any previous century, but we did not see it at all, the camera did," he says.

'Hunch'

Hockney's epiphany came about when he visited an Ingres exhibition at the National Gallery in London.

Inspired by the tiny perfect drawings on show, he blew them up "on a hunch".

"I noticed that the lines reminded me of Andy Warhol, who would project a photograph and trace the line and to my surprise some of these had a line similar to that," says Hockney.

He came to the realisation that Ingres must have used a lens to magnify a person, trace the image and then complete the portrait.

Hockney is not dismissing the great masters, merely demonstrating that they used these techniques to enhance their genius.

The real news is the power of the photographic image is coming to an end

David Hockney

But he does assert that his "hunch" has brought the great masters down to earth.

"They are not demi-gods way up there," Hockney has said. "They are marvellous artists but their techniques have a great deal to teach the artists of today."

Some questions go unanswered in the documentary.

Compelling

Where, for instance, are the lenses that these artists used? And why has nobody had an inkling of this before?

Still though, Hockney's evidence is compelling.

Meanwhile at Covent Garden, Hockney's vivid sets for Die Frau Ohne Schatten, created in 1992, are on display again.

Hockney is an accomplished draughtsman

They provide the backdrop for a revival production conducted by Christoph von Dohnanyi and directed by John Cox.

Mr Cox says that Hockney's creativity and willingness to expand his horizons was what made him a perfect choice for the opera.

"This opera is often doen in a colouless way with the priority being to concentrate the mind and certainly not to please the eye," he says.

"This is an opera about the importance of creativity to the continuation of our culture and Hockney is a man who works in colour with intense creativity."

The 63-year-old artist has never shied away from bending his metier to new and unusual uses.

He has always been fascinated by the technology of modern image-making, from software like Photoshop to photocopies, faxes and polaroids.

Now Hockney claims that the technology can be taken out of image-making, or at least the artistry of image-making can be enhanced.

He says that about 150 years ago the photographer usurped the artist as maker of mainstream images.

Manipulation software

Up to that time, he says, the artist used technology like lenses and optics to help create a masterwork.

When photography came along it took the artist out of these images, but Hockney claims that with the technology of photography manipulation software, the hand is back in.

"The real news is the power of the photographic image is coming to an end," he says.

According to Hockney, when you squeeze an image, take something out of it, put something in - you are drawing on the photograph, just as the old masters did.

Hockney's dramatic sets enhance Strauss's opera

Photography, he says, is actually moving back toward drawing and painting.

Suddenly Hockney's foray into the old masters makes sense.

This quintessentially modern artist has traced a way from Van Eyck and Holbein through Andy Warhol and on to the computer manipulation of images.

As Hockney himself says: "There are exciting times ahead."

Secret Knowledge: Rediscovering the Lost Techniques of the Old Masters by David Hockney is published by Thames & Hudson.

An Omnibus special is screened on BBC Two on 13 October.