Where a pencil or a brush can caress the figure of a loved one, a camera can only snap at it. Indeed the language used by photographers of their art is often one of subjugation.

"People are very naive about photography," said David Hockney last Tuesday, at the press conference to launch an exhibition of his drawings at the Royal Academy. He was responding, in thinly veiled fashion, to the news that a newsreader had been interviewed in connection with nude pictures taken of her seven-year-old daughter. Hockney's indignation was understandable but the implications of what he went on to say were a little puzzling. Pointing out that he encountered few "warm depictions of a human being" these days, he noted that "the person who was trying to make warm depictions of a human being was arrested by the police". He then announced that he had in his pocket a picture of a little girl in "a highly provocative pose". He was referring to a postcard of a Fragonard painting, in which a young girl is shown naked, legs drawn up to her chest and a small lap dog dangling between her knees, its tail brushing against her genitals.

There was one very odd thing about all this, something that seemed to be overlooked in the fuss over the fact that Hockney had intervened in the matter at all. The oddity is this - that an artist of Hockney's stature should, even by implication, suggest that there was no distinction between photography and drawing. "The desire to make depictions of a human being is a common thing, part of a great tradition in art," he said, as if there was some unbroken continuity between Rembrandt and domestic snapshots. He even called on the Royal Academy to make a stand on the issue, as if taking pictures of children in the bath was now part of that venerable body's sphere of influence.

It is odder still because Hockney himself knows that there are sharp qualitative distinctions between the two media. He had virtually said as much the day before, when he was interviewed on Start the Week. "You have to deal with drawing," he said, "because everything else will be a photographed image, and photographed images get a bit boring after a while." He might have said much more. Imagine, for a moment, that the postcard he'd drawn from his pocket had not been a painting by Fragonard, but a photograph, taken in a bed-sit by an off-duty supermarket manager using an underage model. The subject matter might be identical but its implications would be very different. And it isn't just that in the case of the Fragonard " 'Twas another country and besides, the wench is dead". Fragonard may have used a model, but to be painted and to be photographed are quite different experiences. A painter makes a picture, a photographer "takes" it.

The point is made in Hockney's own exhibition, which includes several portraits of naked young men, genitals exposed. But there is a large difference between Hockney's drawings of his friends and the sun-splashed beefcake that first lured him to California, even if the subject matter is similar. These are tender and intimate images and their tenderness is inseparable from the manner of their making. Were these cheap photographs, the gaze of the subject would stare back, not at the artist, a known presence in the room, but out at unknown consumers, waiting to ease their appetites. Instead of a form of intercourse between two people you would have only masturbation.

These distinctions create a sharp divide between the two ways of making "depictions". Where a pencil or a brush can caress the figure of a loved one, a camera shutter can only snap at it, even if, as in Hockney's Polaroid collage of his mother, it takes repeated affectionate nips. Indeed the language used by photographers of their art is often one of subjugation - look through the anthology of quotations at the end of Susan Sontag's On Photography and the verbs leap out at you - "seize", "trap", "arrest", "imprison". In all cases, the fugitive is secured, sometimes with violence.

What's more, drawing is less constrictingly tied to reality than photography. Elizabeth Barrett shocked her family by saying that she would rather have a photograph of a loved one than "the noblest artist's work every produced". She understood that what you sacrifice in artistry you might gain in fetishistic power: "the fact of the very shadow of the person lying there fixed forever!" When Dawn French was the subject of a television profile recently the film included a full-length nude painting of her. She was perfectly happy to have this displayed on national television but would, I suspect, have felt very different about stripping for the cameras. She needed a veil of artistry between her private self and the world.

Some people are naive about photography. But it's surprising to find David Hockney among them.

David Hockney is reviewed on page 4