You may think that a whole load of photographs of people you don't know would be about as boring as the album of a family you have never met. But far from it. The annual photographic portrait exhibition, which (sponsored for the first time this year by the law firm Taylor Wessing) opens on Thursday, has long been a high point of the National Portrait Gallery's calendar. The faces of strangers can clearly fascinate.

Perhaps it's an inbuilt biological response. From the moment we are born, we are programmed to be stimulated by faces. We have to be able to pick out the patterns of our parents' eyes, nose and mouth. But the fascination goes deeper. When we look at a picture of someone, we are not just looking for an accurate description of shapes and proportions (though the way in which the face can come up with such an incredible variety of different looks is astonishing enough in itself). We are looking for a picture of a psychological life.

Will we find it in the Taylor Wessing Photographic Portrait Prize 2008? The images in this show are nothing if not striking. They have to be. More than 6,000 pictures by professionals, students and amateurs passed before a judging panel, which this year included the photography critic of The Times, Joanna Pitman. It certainly helps to submit something eye-catching.

Only 60 pictures go on show but their range is extensive - if perhaps a little self-consciously politically correct. From birth to death, from the wrinkled old pensioner to the overpainted dolly bird, from the African boxer to the Arctic herder, this collection presents a kaleidoscopic picture of humanity. But, what they all share is a daring way of seeing that commands our attention.

Of course, there are some obvious ways to make people look. A handful of celebrity sitters pop up, from Vladimir Putin through Barry Humphries (Dame Edna Everage) to Doris Lessing. That lovely svelte girl who, wearing nothing but a thong, puts on her make-up in front of a mirror, has only one leg. That frail old man has a concentration camp number tattooed on his forearm. We search his face for the scars of a never-forgotten suffering.

For a portrait to work, such eye-catching tactics must be far more than mere tricks. Once we have been made to look with the same intensity as the photographer looked, we must discover more: something more personal, more profound, more provocative; some political outlook or emotional reality or universal truth. This is the deeper fascination of these pictures. Here is Pastor Phil, for instance, the Reformed Baptist Church preacher. His great belly seems almost to barge out of the picture. Like the king in some medieval picture, he seems over life-sized. We sense his horrible bullying power as he stands, Bible in fist, loaded gun in belt, a great bruiser of a bouncer standing at Heaven's gates.

Or there is Ines Connected with Amina, a portrait of two teenagers who, just as saints carry identifying attributes, clutch the iPods and mobiles that are the totems of their culture. This is more than just an imaginative composition showing two pretty girls. It is a portrait of society. Faces lit by the glow of computer screens, they slump side by side, apparently conjoined by a network of wires. And yet, they inhabit utterly different worlds. They don't look at each other. They are too tapped into technology. A little electric connection light glows in one corner. It reflects like a focal point in the pupils of eyes.

The most remarkable portraits in this show range from the eloquent simplicity of Hendrik Kerstens's Bag, a portrait of his daughter like some modern-day Vermeer model wearing a translucent plastic bag like a traditional Dutch cap on her head, to the slick elaboration of Lottie Davies's Quints, in which a woman reclines in a richly swagged boudoir, five babies squirming about her like piglets.

Of course it's not real. It doesn't pretend to be. You can't have five babies at once and still look as serene as an odalisque. The woman is a model. And it's the same baby photographed five times over. But this picture still captures a truth. It is part of a series in which Davies sets out to make photographs of her friends' dreams. Digital technology can capture fantastical scenarios with that disturbing illusion of veracity that a dreamer feels.

Quints is a carefully composed piece of work, not a lucky snap. Several contributors to this show seem acutely aware that their practice might not be considered fine art. They draw self-conscious parallels with painters. Here are the photographic versions of Renaissance portraits, of Friedrich's Romanticism, of Manet's Olympia and the eroticism of Balthus.

But when a camera comes two-for-the-price-of-one with the ubiquitous mobile phone, can anyone be a photographer? It certainly wouldn't seem so. For all that the selection process is strictly anonymous, almost every picture picked, it turns out, has been taken by, at the very least, a former photography student. And it is not just the high production values that mark out the professional works.

A photograph is far from a straightforward reflection. In deciding how an image should look, in playing with composition and subject and light, in preferring one exposure, one perspective, one angle to another, the person behind the lens is imposing their own particular vision.

A photograph is as much an interpretation of the world as a painting. It can tell you as much about the artist - his political views, for instance, or his social outlook - as it does of the subject.

Artist and model come together to form a shared creative force. When this relationship is intimate, the picture is most revealing. It is no accident that so many of the subjects in this show are family or friends. We need to know someone well to see the authentic face, as anyone who has tried to see his or her “real face” in a mirror or been told to “just be natural” in front of a camera will know. Looking natural in front of the camera is about the most unnatural thing for a human to be.

This makes a portrait such as Tom Stoddart's of Rupert Murdoch particularly impressive. How many people see the worn-out sadness of the ageing media magnate? This is a picture that arose from a privileged relationship. Or look at that group portrait of teenagers in a fast-food outlet. At first glance they look about as real as Bratz dolls. But stay a little longer and the individual characters start emerging and with them our growing sense of empathy.

Gradually, the spectator finds himself entering into a relationship with the person in the portrait. And perhaps that is precisely why the National Portrait Gallery's photographic portrait show is so popular. The best portraits are not just images of strangers. They are pictures of people whom we will get to know.

The Taylor Wessing Photographic Portrait Prize 2008 opens at the National Portrait Gallery (www.npg.org.uk) on Thurs

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