Painting portrait of Dutch masters at their height

You have seen it before. It is one of the most iconic paintings of the 17th century. But until you stand in front of Rembrandt's The Syndics (De Staalmeesters) and look directly into the eyes of the elderly gentlemen who break off from poring over their accounts book to look up as you interrupt their meeting, you do not understand its power.

There is no dramatic event to focus upon here. It is a static painting of men whose job it was to monitor the quality of dyed woollen cloth produced in Amsterdam. A "non-moment", Betsy Wieseman, curator of the Dutch Portraits exhibition at London's National Gallery, calls it, but made into something arresting and momentous by Rembrandt's clever use of composition and light.

Rembrandt infuses it with drama, a bit of suspense, she says, "adding energy and movement into the composition by having one man rising from his chair . . . also Rembrandt's unusual use of light: both subtle and dramatic at the same time." It is large and deliberately ostentatious - we look at it from below - and both picture and men exude power and confidence. In an almost insidious way, it grows on you.

This was Rembrandt's final group portrait, painted in 1662 some 30 years after another highlight of the show, The Anatomy Lesson of Dr Nicolaes Tulp. In this somewhat macabre painting, Dr Tulp explains the musculature of the arm using the corpse of a convicted felon (a common practice) to a group of fascinated medical professionals. A less rapt group of surgeons in Nicolaes Pickenoy's adjacent work, The Osteology Lesson of Dr Sebastiaen Egbertsz (1619), show a touch of irreverent humour as one tickles the rib of the demonstration skeleton.

This is the first major exhibition to trace the development of Dutch portrait painting during the 17th century, a period when the Dutch Republic experienced an era of unprecedented wealth following independence from Spain in 1581. During this Golden Age', power and patronage transferred from court and church into the hands of a new Protestant middle-class elite. As a result, merchants became the principal patrons of the arts, and to establish and reinforce social position wanted their likenesses made and important moments in their lives commemorated: births, marriages, professional and civic appointments. Artists were forced to find solutions in portrait painting to satisfy their new clientele's evolving demands. Thus, the northern Netherlands in the 1600s produced an unprecedented number and variety of portraits, demonstrated by 60 works in this exhibition, painted between 1599 and 1683.

Rembrandt and Frans Hals, although undoubted masters of the genre (the exhibition includes nine Rembrandts, and a dozen by Hals), were not the only painters who made portrait production their stock-in-trade. Tens, if not hundreds, of others did the same, and 29 of them are here, some little known in the UK and unrepresented in UK public collections.

Change was in the air; informality too. The exhibition makes this clear, even to non-expert eyes: you only need be in the opening room to see it. Straight away, preconceptions of Dutch art are challenged. For example, the picture that greets you is Jan van Ravesteyn's Portrait of Hugo Grotius from 1599, the earliest on display. This is a highly unconventional portrait, in close-up circular format and oblique pose, of a precocious 16-year-old lawyer, poet, ambassador and justice.

Some changes must have seemed revolutionary back then. In the first room, are double portraits of mother and daughter, father and son, finely painted in 1602 by Jan Claesz: staid formal poses, they are typical of their time. Compare this to the laid-back marriage portrait across the room painted only 20 years later by Frans Hals. The Double Portrait of Isaac Massa and Beatrix van der Laen positively radiates informality despite sober black clothes and her stiff millstone ruff. The couple lounge against one another, completely comfortable, smiling, satisfied, in a symbol-strewn garden of love.

The large-scale group portraits of members of charitable institutions and civic guards - the so-called militia pieces - may be more familiar. By far the largest here is by Frans Hals and Pieter Codde (1637), incorporating 16 full-length portraits of the Officers and other guardsmen of the 11th district of Amsterdam under the command of Captain Reynier Reael and Lieutenant Cornelis Blaeuw, known as The Meagre Company' (to give it its full title).

Artists taking on commissions like this faced difficult, if lucrative challenges: as well as having to produce individual likenesses and get the pecking order right, they had to convey a sense of the group ethos. Hals evidently did not perceive a problem taking on this commission in 1633, but proved too wayward for the company of crossbowmen and was dismissed four years later in favour of a local artist.

Hence, it is a painting of two halves: on the left, portraits by Hals; on the right, seven figures by Codde, wooden to some eyes but successfully rendered according to curator Wieseman, given that Codde, a man used to small-scale pictures, had to accommodate to the energy and personality of Hals's distinctive style. These works embody civic duty and splendour. Civic pride bursts from them, stepping forth, chest out, waving its standard. But homelier works had their complement of pride too.

Conjugal pride and the pleasure of possession shone through in popular marriage portraits; parental pride likewise as families made concerted efforts to show their children (infant mortality was high): see the twin infants painted by Salomon de Bray nestling together in a Baroque shell cradle, and the loving intimacy between the infant Catharina Hooft and her (unnamed) wet-nurse in Hals's charming portrayal.

From the finely detailed faces of Rembrandt's Syndics, to his wonderfully informal picture of a stout elderly man, loosely-clothed and ruddy-cheeked, who looks for all the world as if he's just woken from a nap, through the sparkling brilliance of Franz Hals, and a cast of other characters, this is a gem of a show. It runs until September 16. before travelling to its collaborating gallery, the Mauritshuis, The Hague, where it opens in October.