Canaletto and his Rivals, National Gallery, London

Why these stiff, theatrical views spawned so many imitators is a question unanswered

One of the oddities of Venetian Renaissance painting is the relative absence from it of Venice.

If your studio looked out on, say, the Giudecca, you'd think the urge to paint it would be overwhelming. Yet, contrarily, the triumph of Giorgione lay in dreaming up a terra firma landscape, of the Bellinis in depicting a new sacred mood, of Titian in a mythological world made real. If this strikes us as perverse, then that is because Venice has, in the past 300 years, become a byword for the urban picturesque. And for that we have to thank one man, his name, by happy chance, being Canal.

Giovanni Antonio Canal, known to all but the logical Germans as Canaletto – they tend to save "Little Canal" for the master's nephew and pupil, Bernardo Bellotto – probably did more than any other artist ever to print the single image of a city on the public mind. If Venetian painters of the 16th and 17th centuries had looked outwards, Canaletto turned his gaze firmly inwards. How this happened is the subject of a new exhibition at the National Gallery called, baldly, Canaletto and his Rivals. The question that remains largely unanswered, though, is why.

It's a problem that first struck me a decade ago, coming out of the cloister of San Giorgio Maggiore. Inside was a show of Canaletto's drawings, including several of the church itself. Many of his vedute – vistas – are either of Palladio's masterpiece or painted from it. (Apart from a coincidental glimpse of San Giorgio in the Bacino di San Marco, none of these pictures is in this show, even though there's a handily borrowable one in Manchester. I'm not sure why.) What was striking about Canaletto's drawings of the scene was how lively they were – loose, freehand, full of the noise and movement that hit you, then as now, as you step out of the basilica's Benedictine calm into the milky Venetian sun.

By contrast, Canaletto's paintings of (and from) San Giorgio are glacial: not so much lifeless as artificially alive, acted out in a stagey way that seems to betray the artist's training as a painter of theatrical backdrops. In looking outwards – to dry land in the case of Giorgione, Classical antiquity in the case of Titian – the painters of Renaissance Venice were like their fellow citizens: acquisitive, curious and mercantile. Theirs was the Venice Shakespeare would imagine for Shylock and Portia 20 years after Titian's death. Canaletto's painted city was more like Racine: ritualised, decadent, ancien régime; a city-state doomed to die.

So why did Canaletto's image prevail? Although the National's show is full of good things, it doesn't answer this question. There is a great deal about who followed whom, and a certain amount about how. What is missing is any real suggestion of why – why an image such as Canaletto's stiff, theatrical, box-spaced The Piazza San Marco, looking East, on loan here from Madrid, should have spawned all those rival vedutisti, the Marieschis and Bellottos, the Cimarolis and the Guardis.

The obvious answer to that – that Canaletto's work was the favourite choice of English milordi on their Grand Tour looking for luxury snapshots to take home – doesn't really solve the problem. Which came first, supply or demand? What was it about Canaletto's unlively imagining of Venice that made it so saleable to foreigners? More, why did a man who, when he drew, drew a living, breathing city, paint it with all the rigidity of a Japanese print? You'd think, from works such as the Bacino di San Marco on Ascension Day, that these canvases were made in the studio – squared-up, probably, from rigidly accurate sketches. Actually, Canaletto pre-empted the Impressionists by painting en plein air, setting up his easel in piazzas. And why do we hardly know the name of Michele Marieschi when his looser brushwork – The Rialto Bridge from the Riva del Vin, on loan from the Hermitage, is a case in point – seems so much more modern than Canaletto's?

The main flaw of the National Gallery's show, to put it briefly, is that it treats Canaletto as normal. So used have we become over the past three centuries to his being the default image of Venice that we assume that it was inevitable that it should be so. Personally, I'd like some explanation of how this inevitability came about. Canaletto had rivals for the reason that Microsoft has rivals, because he was very, very successful. But why?

To 16 Jan (020- 7747 2885)

Next Week:

Charles Darwent squares up to The Glasgow Boys at the Royal Academy