The first thing you see on entering this major new Viennese exhibition is not one of Canaletto and his nephew Bernardo Bellotto’s majestic paintings of London, Venice or Vienna, but a camera obscura. The magical art of both artists depended upon this simple but effective device, which exploits pin-hole projection – an optical phenomenon that had been known since antiquity. 

The decision to open the show at the Kunsthistorisches Museum with a deceptively boring little wooden box amounts to a curatorial throwing down of the gauntlet. Because – although I find it hard to fathom – there are still art historians and critics out there who refuse to countenance the fact that great artists used optical aids. The show follows hot on the heels of the publication last year of Philip Steadman’s Canaletto’s Camera (UCL Press) in which he demonstrated precisely how the Venetian master employed drawings made with a camera obscura to create his paintings. (Canaletto’s own – stamped ‘A. Canal’ – is on display at the Museo Correr, Venice.)

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A pin-hole image appears upside-down and back to front, but both these aspects can be adjusted in a purpose-built device through the incorporation of one or more mirrors (you can try out a sophisticated 1828 camera obscura at Bristol’s Clifton Tower). Canaletto’s own little box showed images the right way up although still back to front. The scene before him was projected on to a ground-glass screen – shaded from the light by a wooden hood – where it could readily be traced on to transparent paper, which was then simply reversed. 

After the second world war a sketchbook was presented to the Accademia in Venice that contained no fewer than 140 pages of sketches by Canaletto of Venetian buildings, all of them related to finished paintings. The pages are tiny, 17x23cm, limited by the size of the camera obscura. A few are very rough initial outlines, but the rest are highly detailed, many of them worked over in ink and even annotated with shop names. Owing to the petiteness of the device, Canaletto had to take a number of sketches of any particular view,........

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