Canaletto's panoramic paintings of London are awesome. I've seen some of his stuff at the National Gallery before, and always thought that they resembled architect's technical drawings (this is a good thing). At the Dulwich Picture House's exhibition of his work in London between 1746 - 1755, we get to see the preliminary sketches alongside the finished works, and this apparent scientific accuracy is on display more than ever.

Or is it? One of the continuing themes of the exhibition is Canaletto's looseness with facts. Sometimes this is geographic and architectural, in terms of prettifying the painting's compositions. Elsewhere it means bending to the will of his purchaser, emphasising a building or person representing his benefactor. Canaletto also took the unusual step of producing pictures on spec, so occasionally the filling in of landscape blanks was purely practical in nature, and such details anyway were usually sourced from architect's or builder's plans, as in the case of the pedestrian shelters on Westminster Bridge, which he painted but which never materialsed in the finished version of the bridge.

The huge landscapes are instantly impressive to the eye in size and scope. The low horizon, disproportionate amount of vast, empty sky and weighty mass of detail in the bottom third of the frame make the Canelettos an undoubted influence on Nigel Cooke. They are teeming with life when viewed close up, a truly personal view of city life at odds with the amount of minutely rendered bricks and mortar we see initially.

The golden light, the clear blue skies and the beautiful, yellowy, Venice-esque brickwork are all hopelessly idealistic, undoubtedly a plus point for the buyer. They resemble postcards. In fact they would probably have been many people's first view of London, and I wonder if they lead to hordes of disappointed European visitors arriving in a wet, grey West End asking for their money back.

Canaletto's paintings outside of the metropolis are less exciting, containing as they do wide expanses of green pasture from the suburbs. He was a city boy, and even in Hampton Court or Walton-on-Thames he concentrates on architectural features such as newly engineered bridges. In fact in 9 years in England he never ventured further than about 150 miles from London.

A good and thorough showing of the Italian artist's time here. Crammed with well-annotated pieces, I can recommend setting aside at least one and a half hours for the Canaletto exhibition, and if you have some spare time you may want to check out the Picture House's particularly well-executed Mona Lisa copy, which the owner beleived to be the original for many years.