Now in its 29th year, the National Portrait Gallery's BP Portrait Award is one of the art calendar's more eclectic fixtures. Often derided by critics and yet resolutely popular with the public, it is also an exercise in artistic democracy - up to a point, at least. The final selection is a matter for the panel; but absolutely anyone over the age of 18 can enter. Unlike more grandiose arts awards, the raw material does not emerge purely from a self-selecting group of elite artists. In theory, at least, the likes of you or I could land wall space in the final exhibition.

In practice, of course, this is not the case. Almost all the 55 artists in the final selection have form - there are no X Factor moments here, no ordinary people with unmined seams of genius. Craig Wylie, whose vast painting of his girlfriend Katherine is one of the four shortlisted for the prize, is well-known on the art circuit, and this is his fourth year in this show. Ditto Paul Benney, whose stunning Blue Pool catches the eye the second the viewer steps foot in the gallery.

There are also some famous faces on the other side of the canvas: the film director Mike Figgis, lost in a moment of musical rapture; Sir Jeremy Isaacs, festooned (somewhat tiresomely) with the symbols of his success; and a small but extremely intense Carlos Acosta. Despite the familiar, however, there is a striking lack of formality to almost all these portraits. If there are any artists of the ponderous, pompous school of portraiture left, they did not make the March deadline for submission; or if they did, they were mercifully culled before the final cut. These are not works addressed to posterity, destined to grace the walls of boardrooms and mock-Tudor drawing rooms. They are very much of the here and now, fleeting moments that somehow tell us something about the subject's entire existence; personalities captured in a single glance, lives expressed in a snapshot.

The influence of photography generally on the art of portrait painting is undeniable, and here it is unavoidable. The walls are full of flashbulb moments, faces and bodies that stand still barely long enough to submit to the camera shutter, let alone the artist's brush. Some of the sitters even appear to be moving between moments: André Hunt's Self Portrait with Winnie, in which the artist pictures himself cradling his five-week-old daughter, pie-eyed and half-dead from parental exhaustion, cradling his five-week-old daughter; Stephen Chappell's studiedly tasteless Ooh Err Mrs!; My Mother-in-Law, Anne, and Her Sister, Auntie Audrey by Tony Noble, a wonderfully funny painting of two very grumpy old ladies, caught in an attitude of fond familial loathing.

There is a degree of confrontation here too. These are not self-satisfied subjects, comfortable and secure in their life's endeavours. They challenge the viewer, almost daring them to intrude on their intimacy. Angela Reilly's arresting self-portrait is a fine example of this, an uncompromising, unflinching assessment of the artist's face devoid of artifice or make-up. Wylie's K has an air of accusatory, almost pleading exhaustion to it. The sitter's sullen expression contrasts starkly with her cheerful pink cardigan, rendered in meticulously high definition. It feels as though, faced with the complexities of his lover's face, Wylie has taken refuge in the equally intricate but far less emotionally demanding threads of the garments beneath.

The theme of age and ageing, already touched upon by Reilly, is strongly present elsewhere in the show. Nicola Green's limpid and distinctly spooky portrait of a small boy and his toy monkey captures perfectly the blankness of a child's face in repose. Oil paintings of children are fiendishly difficult to carry off. The sophistication of the medium seems almost too much for such a simple subject: children's faces are free of complex emotions, untroubled and unmarked by life's trials. They are very hard to paint without looking chocolate boxy, but this Green has somehow achieved.

At the other end of life is Hannah O'Brien, Robert O'Brien's grandmother, and the subject of his tiny but intensely powerful portrait. Hannah's face, melting with extreme age, has a painful quality to it, the heightened reds and browns emphasising rheumy eyes and mottled skin in a confident, bold hand. Her gaze drifts to the left, both bewildered, wise and wondering. It is almost as if, approaching death, her physical boundaries are beginning to blur, and O'Brien has somehow, brilliantly, captured this; only her blue eyes - her soul - remain intense in the midst of the wreckage.

There are, inevitably, one or two stinkers and so-whats, not to mention a few why-on-earths. Those portraits that seek to make political - or politically correct - statements are not only irritatingly naive but also counter to the whole point of modern portraiture, which is to subtly represent the sitter's personality, not bash the viewer over the head with it. As the best of this show proves, the epic is in the intimate, not in the obvious.

The BP Portrait Award exhibition runs at the National Portrait Gallery, WC2 (020-7306 0055), from Thurs to Sept 14. The winners will be announced on Monday

Editor's choice

• The 100 Best Films of 2008
• Hitler and the Downfall mash-ups
• Britney Spears: The Circus