Dutch Portraits: The Age of Rembrandt and Frans HalsNational Gallery, London WC2, Wednesday to 16 September  Anyone who thinks Dutch portraits are all black and white and stiff as a Protestant's ruff should look again, specifically at the amazing exhibition that opens this week at the National Gallery. Never before have so many startling images by so many known (and unknown) stars been brought together to give a sense of teeming life and art in the Netherlands. Even the Rembrandts are unexpected visitors to this country, never mind the dozen portraits by Hals. From the tight close-up to massive group shot of militia men posing with all the languor of Vogue models, the show is a revelation from first to last.  Dutch art of the 17th century has tended to mean Rembrandt, Vermeer, the tulip and the chequerboard floor, with the marvellous landscapes of Ruisdael and Cuyp a fairly recent revival. There are exceptions, but the portraits of the Golden Age are long due for rediscovery. Embodying all the energy of a new republic breaking free of Hapsburg rule, these paintings no longer represent kings and courtiers, but the surging middle classes. Every town has its local painter, every profession from banker to philosopher to orthopaedic surgeon wants to get into the picture.  Dutch artists could have just adapted the usual formats - front or side on, half or full length - but instead they went for extraordinary innovations. Jan van Ravesteyn's portrait of Hugo Grotius is a circular close-up of the head turning to throw a glance, ruff rising in diagonal fanfare, eyes full of the quick intellect that made Grotius a controversial lawyer and playwright at 16. Emmanuel de Witte shows Mrs van Heusden and her daughter down by the docks, large as life and Pre-Raphaelite in every detail, haggling over the price of catfish.  Salomon and Jan de Bray, father and son and new names to me, remade the family portrait. The twin infants in Salomon's animated image are cradled in a giant shell and turned slightly towards each other so that you can see the psychological bond between them. Jan paints his parents facing left in profile, one just behind the other so that they appear to be two minds united in a single body and heading in the same direction.  All kinds of portrait are being invented in the Netherlands at this time - the laughing couple, the children's orchestra, the whole family out in the garden. The pleasures of peace are conflated with the new pleasures of wealth. In a fabulously opulent painting by Jan Mytens, Willem Kerckhoven and his wife and 10 children are portrayed outdoors in a line-up that could have been relentless and rigid. But each figure is busy with peaches, grapes or strings of pearls, roses that need picking, jokes to be told, dogs to be soothed, a family romance crowned with a flight of putti just coming in to land and two priceless tulips planted centre-stage to show this paradise is Kerckhoven's garden.  The professional group portrait may be the most familiar of these Dutch innovations - guild members, militia, hospital governors - but even here, the show springs surprises. Nicolaes Pickenoy's Anatomy Lesson, which turns out to have been painted long before Rembrandt's more famous Doctor Tulp, shows bone man Dr Egbertsz, flanked by five other surgeons, examining a skeleton that dangles in their midst. The skeleton, of a pirate executed in 1615, seems to be gracefully sashaying and the heads and hands of the medics, spotlit in deep darkness, are choreographed around it like a mysterious performance: a dance of life and death.  Heaven knows how the National Gallery persuaded the Dutch to lend The Anatomy Lesson of Doctor Tulp but here it is, in all its ineffable strangeness - the doctors leaning over the grey corpse and staring hard at the anatomy book as if it were more real than the body, the figure of Tulp coruscatingly bright in the spectral gloom, the flayed hand turned palm upwards as if making a point, Rembrandt's point if you like: that only we see the dead man as once human.  One of the riveting lessons of this show is that Rembrandt, for all his genius, had no monopoly on the raging market for portraits. People wanted bigger or brighter or less unsettling images, they didn't necessarily want to look like a Rembrandt (or like Rembrandt himself: at least two paintings here could pass for self-portraits). To modern eyes, Rembrandt's Elderly Man may be incomparable - the breath crackling, the flesh sinking into human clay, only the eyes still bright like the last lights in a ruined house. But tastes constantly change and Rembrandt in any case gave up portraits for decades.  Kenneth Clark, for instance, once scorned Frans Hals as 'revoltingly cheerful and horribly skilful'. But Manet loved him and Manet was absolutely right. Hals may be less neglected than any other Dutch portraitist bar Rembrandt, but here are many works never shown before in this country - his affecting paintings of Haarlem pensioners, of sharp-eyed intellectuals tipping back in their chairs, of a new husband and wife fairly cracking up with laughter.  Where others go for flawless form and finish, Hals is always breaking out, recreating the vitality of his sitters with the flashing dabs and irregular spots so admired by the Impressionists. A letter is passed like contraband, a man leans out of the picture, lace explodes like splintering ice: every image begins with a drama kept in play with quickfire brushstrokes.  But more than that, Hals's gift is for the double response, for making it seem as though his sitters are fascinated to see you as well. Round a corner, you come across the surprise of a face that seems to have been waiting there especially for you. Hals works on every scale, from the colossal pageant to the intimate little study; best of all is his tiny close-up of the calligrapher Jean de la Chambre, the tendrils of his hair, the dazzling pleats of his collar so delicately described they might have been drawn with the calligrapher's own ink and feather.  These portraits by Hals would have been tremendous enough on their own, but together they form a show within an even greater show which anyone interested in painting, or people, should try and see. For what Dutch Portraits overwhelmingly reveals is just how far these artists extended the repertoire of portraiture and, thus, the way we see ourselves, for ever.  Helio OiticicaTate Modern, London SE1; until 23 September  Brazilian artist who put the power of colour into sculpture.  How We Are: Photographing Britain 1840-PresentTate Britain, London SW1; until 2 September  Tremendous family album of great photos.  Lisa MilroyIkon Gallery Birmingham; until 2 July  Paintings exploring the quiddities of the garden.
Art: In the young republic of the 17th-century Netherlands, painters - and the surging new middle classes - reinvented the art of portraiture. The results are astonishing, as the National Gallery's show reveals, says Laura Cumming.