The watchers

There's a right length of time for looking at a picture. Some demand lengthy, careful views. The Adoration of the Mystic Lamb in Ghent Cathedral rewards slow appreciation. It's worth sitting before Rembrandt's The Night Watch in the Rijksmuseum in Amsterdam or standing for a long while before the Breughels in Brussels' Musée des Beaux Arts or the classical scenes by Claude Lorrain in the Louvre.

Even some portraits are made for time-consuming adoration. These are usually pictures of the powerful, commissioned to demonstrate their finery as well as their faces. But the poor as well as the rich can seem in control of their images. Murillo's street urchins often display the confidence of the young and free. Even Goya's Maja, clothed or naked, seems in control of how we see her – as does Lucian Freud's model Sue Tilley, the Benefits Supervisor.

Elsewhere the subjects of paintings see less happy with the viewer's gaze. There are nudes by Titian whose lowered or averted eyes can cause discomfort. And I can never look at portraits or even staged domestic interiors by Vermeer for long. The framing of the paintings always brings me slightly too close – I'm forced into the scene like an intruder, as I am.

This history of looking was at the back of my mind as I dithered in the entrance hall of Tate Modern. I couldn't make up my mind about the new exhibition. Not only was the entrance ticket £10 – very high by East Midlands standards, especially for a show of photographs and videos – but the subject itself made me wonder whether I ought to look. The exhibition is called Exposed and the material it shows is grouped around themes of voyeurism and surveillance.

At first I decided I wouldn't look. I'd taken the long escalator to the third floor when I changed my mind and returned to the entrance. The escalator journey felt too much like running away.

There was, of course, something uncomfortable about so many photos grouped around such a theme. Few of them on their own would have sparked such concern even though some areas were banned to under-eighteens because of their sexual subject.

But sex and violence are too familiar to shock. Even the napalm picture, with the running, naked girl (her clothes burnt off) in a world beyond fear and pain, has been reproduced so often that I'm now immune to the horror. I was more moved by the photo of Jackie Kennedy with the new president Johnson and his wife. She seemed numbly vulnerable as she took on the unfamiliar role of presidential widow – beyond comfort and open to the camera's exploitation.

Elsewhere too the camera's glance was surreptitiously controlling. I was disturbed by Walker Evans' pictures of travellers on the New York subway, some tired at the end of the day and enjoying private rest in the carriage's public space. The photos were taken, before the Second World War, with a camera concealed in a coat-sleeve so that no-one could object. All the subjects must be dead by now. For all I know they would have liked the idea that gallery visitors would pay the unbelievable sum of £10 to see their faces – it's a kind of immortality.

Having seen the subway photos, I was initially suspicious of the work of August Sander as he posed and classified “types” - by profession, disability, social status – in his project to document the layers of German society. But as I looked at the individuals he photographed with such care, I warmed to him. The pictures seemed to be the product of a kind of negotiation between photographer and subject. Most of them gazed back at the photographer with the confidence of people who had chosen their pose. There were exceptions of course: the boy labelled “idiot” or the blind girls, sitting together at a school desk. They were photographed in the mid 1920s and despite the labels the children appeared individuals. I wondered what happened to them after the Nazis came to power. Sander's sympathetic categorisation reminded me of the Nazis' brutal hierarchy of types. Yet the Nazis disapproved of Sander's project. His son Erich, a socialist, was imprisoned in the early days of the Nazi regime. After many years, Erich died in prison. Sander photographed his son's death mask.

Of course the exhibition disturbed – it was meant to. But it was far less disturbing than the film I saw two days before, Renzo Martens' Episode 3: Enjoy Poverty.

Martens' film explores poverty in the Congo – the sort of daily, agonising poverty that causes children to die painfully of slow starvation. But it's not like the kind of TV documentary that ends with an appeal to send money, offering the comforting, self-congratulatory reflection that we in the west are nice, charitable people. This begins with a man in the Congo complaining about poverty as he works and angrily showing his small daughter's sores – a result of malnutrition.

At first the film seems a tourist's journey through poverty in which Martens' occasional efforts to understand what he sees don't measure up to the reality. He seems too innocent, too polite. But the guns of the U.N. peacekeepers disturb as do the sharp suits of the wealthy.

But it's easier to look at the powerful than the underpaid workers whose children die so that people like me can buy goods cheaply. And then there's the war and the photo-journalists who follow war and starvation, looking for a scoop. They cluster together with their expensive, heavy cameras, each trying to find the best lighting and angle for a picture of a corpse. The going rate for a publishable picture is $50, on top of the travel and expenses already paid to the photographers. Martens, playing the innocent, asks a photographer a series of questions. Who owns the picture? Do the people in it – or the relatives of the corpse - earn money from it? What pictures sell?

Of course, it's the photographer who profits. The poor children, who aid workers strip and arrange for the camera, get nothing. So Martens reaches his conclusion. Congo has few assets apart from poverty. If the people of Congo are to profit, they must learn to exploit their own poverty. He finds two Congolese photographers, making a small living from pictures of festivals, birthdays and weddings, and urges them to change their subject, telling them that the highest profits come from photographing the dead and injured, raped women and starving children. I'm used to seeing such pictures but I've never before had to sit uncomfortably still while the subject's pose is adjusted and the photographer closes in for the most affecting and aesthetic angle.

There were questions and answers after the film. Martens insisted his work changed nothing – it was art, not politics. But he made a telling comparison with Jonathan Swift's “Modest Proposal,” identifying the film as satire based on a horrible truth. And he said there would be no change because there was no will to change. The people in Congo thought that if those on the outside saw what was happening, they would act – they would not allow such suffering to continue. But there is no outside – we're all in the same world and those of us with power to act do nothing because we benefit from the same system.

Meanwhile, for reasons we cannot bring ourselves to comprehend, pictures of pain and suffering are bought in the west as though they were things of beauty. And the sufferers get nothing for them.