Never has an artist been more fused with his city. All first visitors to Venice familiar with his work experience déjà vu. The Doge’s Palace, the Piazza, the Salute: the new arrivals already know them well, from Canaletto’s paintings, just as they also know that characteristic blue sky. Although Venice nurtured many first-rate artists, Canaletto is Venice. The city called herself ‘La Serenissima’. Canaletto is her most serene painter.

An exhibition entitled ‘Venice: Canaletto and his Rivals’, currently at the National Gallery in London, is important, enjoyable and not to be missed. At a time when London is resounding to the wails of meretricious pseudoartists, terrified lest the public purse should cease to fund their mediocrity, the National Gallery reminds us that public patronage does have a vital role, as a hand-maiden of true art. But there is a paradox. In the case of most major artists, exhibitions enhance their standing. As the succession of canvases delights the eye and overawes scrutiny, there is renewed reverence and increased respect. Not on this occasion – or at least, not with Canaletto.

He achieved mastery very early. The Stonemason’s Yard (c.1725) is a great painting. The less well-known San Cristoforo, San Michele and Murano from the Fondamenta Nove (1725-30) has immense power and promise. By the time he was twenty-six, Canaletto had produced a number of masterworks which earned him immortality. Thereafter, he rarely reached the same heights. It may be that commercialism was to blame. By the eighteenth century, Venice was well into decline. Once mistress of the seas, she was now the madama of an elegant bordello. The once-great entrepôt had become a tourist resort. Tourists seek souvenirs. Canaletto caught their eye. In particular, he formed a virtual partnership with Joseph Smith. Smith was a banker who settled in Venice to become a patron of the arts and the honorary British consul. He discovered Canaletto, which might not have been in the long-term interests of the painter’s art.

There was a problem. Canaletto was easy to sell. A succession of English aristocrats visited Venice, with an unlimited appetite for his paintings. Two centuries later, as confiscatory taxation plundered their descendants, some of the canvases moved further west, to Ottawa, Boston, Washington, Dallas. A thesis awaits a writer: ‘The patrons of Canaletto and the impact of imperial overstretch’. Whatever the longer-term fate of his oeuvre, Canaletto could find an easy market for everything he produced. The good may have been the enemy of the best: quantity, of quality: repetition, of originality. The artist then succumbed to another temptation. As his customers were great English noblemen, why not move closer to them? From 1746, Canaletto spent nearly ten years in England. That may have been good for his bank balance; it was deleterious for his genius. His English paintings are easy on the eye, but the full dramatic flowering of the early promise is no longer to be seen (this exhibition is exclusively Venetian).

There is a further paradox. Canaletto had a nephew, Bernardo Bellotto. As so often in Italy, heredity and apprenticeship helped talent to flourish. Canaletto’s own father was a stage designer – an appropriate vocation when all Venice had become a stage. Young Bernardo Bellotto joined his uncle’s workshop. He also showed early promise. By the age of twenty, he was a worthy rival to his uncle. This was not enough for the tourist market. It may be that Joseph Smith and the Italians were being too cynical and that the English grandees could have been persuaded to recognise a new star in the firmament. But the marketing men were risk-averse. Early on, Bellottos were sold by false pretences as Canalettos. Yet the purchasers’ posterity has no reason to regret the deception. The nephew had no need to take shelter behind the uncle. Bellotto was il miglior fabbro. As they were both painting in Venice, the two men sometimes addressed themselves to the same subject matter. This exhibition juxtaposes some of the results, always to Bellotto’s advantage – with one exception. Canaletto enjoyed painting dogs: something else that might have drawn him to England. They often turn up in his paintings – jolly little fellows riding in a gondola or scuffling in a piazza. Bellotto also went in for the odd canine: never as successfully. His heart was not in it.

But Bellotto rose to other challenges. In his youthful years of unlimited potential, Canaletto had used his paintbrush to grip his subject matter. The scenes he painted were available to the casual glance of anyone who walked past them. In most cases, they still are. But it is as if he had reprocessed them all through his eye, his intellect. This is the highest achievement of representational art. The landscape, the face, the cityscape are universally accessible. The artist puts the unique stamp of his genius on his version. That is how Canaletto started. As he grew older, he slackened his grip.

That was never true of the nephew. Bellotto gives the impression that he was cleverer than his uncle. He certainly had a more restless intellect and was less easily satisfied. He was drawn to a darker palette, as if he would find more challenge in shade than in sunlight. While based in Venice, he also painted in Verona. There is a magnificent view of the Church of San Zeno, now on loan to the National Gallery of Scotland. At the risk of sounding disrespectful to Edinburgh, a major Bellotto is one of the few gaps on the walls in Trafalgar Square. This might have been rectified in the early seventies, when the painting went through the sale room, but the London gallery had been starved of funds by that misery-gutted philistine, Ted Heath. In the Verona masterpiece, the sky is darkling. Bellotto decided to follow the dark. Within a few years, he too moved north, inter alia to Dresden, Munich, Vienna and Warsaw. He was also drawn by aristocratic patrons, but unlike his uncle, he did most of his finest work away from Venice. Although there is no shortage of sunshine in any of those northern cities during the summer, it is as if the brooding skyscapes of the northern autumn spoke to his soul, while his uncle warmed in the caresses of a southern sun and an azure sky. Bellotto’s journey was Gustav von Aschenbach in reverse.

Even though Venice was his birthplace rather than his chosen artistic territory, Bellotto comes well out of this exhibition. It also benefits from the works of minor artists, who helped to shape the Venetian vedute tradition without establishing mastery. But two other artists, the first unexpected, can make greater claims. Michele Marieschi (1710-43) had a double misfortune. A follower of Canaletto, he was easily overshadowed, even before he died young. Over the centuries, he has often been dismissed as a cabinet painter. So he sometimes was; he also had unfulfilled promise. The two views of the Rialto, on show here, are well above cabinet painting. It is easy to see why the one that now hangs in St. Petersburg was attributed to Canaletto for many years: poor Marieschi. The painting falls short of genius. The buildings on the right of the Rialto are in something of a jumble. Bellotto or Canaletto would have found a way to put that right, without sacrificing representational accuracy. But the Marieschi has one further advantage. Like so many other works from the Hermitage, there is a depth of oil on canvas. Soviet communism had one accidental merit: the galleries were too poor to ruin their paintings by over-cleaning.

Francesco Guardi (1712-93) was never in danger of being degraded to cabinet rank. His work is uneven, but at best, he is superb. There are some outstanding examples here. While it would be wholly wrong to claim – as has been alleged – that Canaletto could not paint figures, Guardi easily surpasses him. Some of his figurative work is Goya-esque. Guardi commanded the Venetian light, pushing on beyond Canaletto to become the second great Venetian impressionist, after Titian, and making straight the way for Turner.

This exhibition fails to vindicate Canaletto’s claims to genius, but that is the artist’s fault, not the organisers’. There is plenty to think about and many delights: old favourites and new discoveries. Almost all of these works were painted for the pleasure of aristocratic patrons, and as Talleyrand would remind us, the eighteenth-century aristocracy understood la douceur de vivre. The pleasure persists.

Venice: Canaletto and His Rivals
is at the National Gallery until 16 January 2011