There are two ways to approach the vast silt of sunflower seed husks covering the Turbine Hall at Tate Modern – out of pure curiosity, knowing only that this is the work of Chinese artist Ai Weiwei, or armed with cold hard facts.  The first (and I freely confess this was mine) begins with the strange visual lure, and with an almost animal desire to walk across what appears to be some kind of grey volcanic shore. You want to hear the husks crunch underfoot, to feel them shift, to gaze down at these millions of tiny particles irresistibly proposing broken shells or grains of sand on the beach.  Trampled, pocked, scuffed, they are (or at least, were, until health and safety suspended this pleasure on Thursday, due to anxieties over porcelain dust) constantly in motion yet too numerous to be altered by the tides of feet. And if you stand not upon them but before them, as is now your only option, they simultaneously resemble an ocean. Fathomless, numberless: each physical experience puts one in mind of time and tide, of infinite numbers of units, husks, people.  Perhaps you might reflect upon the artist's nationality, on the difficulty of ever comprehending the sheer scale of China, of its remote and immense population. You might notice how the shingle seems to tinge the atmosphere grey, how sub-fusc the people look. Perhaps you think of Mao, of mass uniformity.  Or perhaps you notice how spectral the Turbine Hall has become, how this silver-grey sea neutralises the stark industrial void above, or how it doubles as an eerie stage-set. Perhaps you bend down and pick up a seed, discovering that it is both colder and heavier than expected, altogether more like gravel.  Whereupon, you consult the wall-text. It turns out that each seed is in fact a unique porcelain replica, hand-painted in Jingdezhen and fired at 1,300 degrees. Some 1,600 artisans worked for two years to make 100 million husks with a combined weight of 150 tonnes: a mass project, its collective spirit now abroad in London. The imagination runs fast from millions of tiny painted sculptures to thoughts of dismally repetitive labour.  Ai Weiwei chose sunflower seeds because they were a source of food and comfort during the famines under Mao; Jingdezhen was the porcelain capital of Imperial China; 100 million is five times the population of Beijing. Individual and mass, mass production and craftsmanship, eastern food, western consumption: associations inevitably form, but they are not – for me, at least – occasioned by the work itself.  There is the art; there are the facts; they converge in the artist's eloquent statements, available in many different forms on the web (tate.org.uk/go/aww). But no matter how crucial the political context, I am glad I experienced the poetry of Sunflower Seeds first, with all its subtle nuances, before the wall-texts with their crushing insistence on statistics. Art – particularly this art, so open to all interpretations – cannot be read like data.  What you see is not what you see: this is how Ai Weiwei's installation has been described and it might stand for Canaletto too, that marvellous illusionist whose views of Venice glinting in the sun appear to present exact topographical truths.  It is always good to be reminded how ingenious Canaletto was, narrowing or widening the Grand Canal, repositioning or deleting a building, even clambering up to some high vantage point that never existed to see the vista spreading out below. And the object of Canaletto and his Rivals, for all its scholarly attention to half a dozen other Venetian view masters, is, I suspect, to show just how superior he was.  The show starts with a surprise: the young Canaletto is totally free with the brush, his skies anticipating Constable with their broad swipes of paint, and Turner with their melting atmospheres and evanescent mists. There are patches of friable masonry and peeling plaster so beautifully worked that the paint tangibly imitates the exact textures it describes.  It is a long way from the famous razor-sharp views, where you sense the brush and ruler in perfect parallel. Even the locations are off the tourist map. That magical work, The Stonemason's Yard, painted when Canaletto was in his 20s, shows a backwater filled with visual incident – a child tumbling over, a remonstrating mother, a tall broom leaning at an angle, half in and half out of the brilliant light. Bottle-glass windows, linen drying, the stencilled lettering on a wall fading with time as the masons hack at the obdurate stones that will repair the nearby church, it holds life in the moment, and all of it in equal balance, this revelation of Venice backstage.  But Canaletto has more styles than are generally displayed: high-gloss, hazy, strictly geometric, glintingly figurative, softly abstract. Perhaps these are all properties of the city itself, with its ever-changing rhetoric of effects, but this has something to do with the market as well, and how good to see it acknowledged.  For all these painters were competing for the market share in postcards for those who had never seen Venice and souvenirs for those who had. As you walk through these galleries, noticing the same spots from different viewpoints, the same carnival masks and water festivals and shining facades, the commercial enticements are blatant – extra gondolas, pretty girls, the Doge's Palace ablaze with autumn sunlight. Canaletto even shrinks his pictures, obligingly, to fit the traveller's trunk.  But the rivals of the title are not precisely that: some precede Canaletto, others are imitators, specifically his pupil and nephew Bernardo Bellotto; others, such as Guardi with his turbulent scenes, are after a wilder style.  Bellotto sees more shadow, more contrast, more depth in Venice. He paints the stone lions of the Arsenale in a coldly menacing light. He may even steal the show for modern eyes with his lone tower at twilight, humped blue mountains in the distance, an intense clarity to close objects in the silver dusk, a last sail pricking the dark horizon: a picture of stillness masking threat.  What unites these artists is detail: every window, every banner and flag, every tile and column. If Venice cannot be summarised, then its beauty must be miniaturised. This can produce paintings of unbearable density, where there are so many masts or balconies or gondola poles there is nowhere for the eye to rest and the mind starts to rebel. But this is precisely where Canaletto excels. He keeps your eye alive, and moving, by giving everything its strong particularity in the general scene, awarding nothing undue drama or weight: the scudding waves, the red coat, the flash of light catching a weathervane, the hat of a man clambering down into a boat.  This all-overness is an exceptional feat when you think of the place itself: a spectacle of beauty in an endless loop. And what is more remarkable still is that Canaletto's is an art of possibility too, the suggestion that there is always more in the scene than your eye can quite see, that if this gondola were to turn that corner another view would open up, that Venice is inexhaustible.  Of course the possibility of boredom is there too. Canaletto painted sequences of up to a dozen works for his aristocratic clients in England, but presumably never aimed for 50 to be shown at once; by the end of this exhibition, they are starting to cancel one another out.  Yet even massed in such numbers, Canaletto's paintings do not block the view. That is his special grace. He gives you the full beauty of the city unobstructed by ostentatious opinions or style. You will never see Venice as a picture by Canaletto.
Ai Weiwei's Sunflower Seeds in Tate Modern's Turbine Hall has a spectral power all its own, writes Laura Cumming