Philip Steadman has been studying Vermeer's working methods for more than 20 years. With astounding diligence and precision, he has unravelled the mystery of Vermeer's simulation of actuality and shown that the painter habitually used a camera obscura.

His new book, Vermeer's Camera , is one of enormous intelligence. It will be a complete reference work for scholars and students and, by virtue of its clear text and exemplary diagrams, a work with an instantly clear appeal to the more casual reader.

Opinion against the idea that Vermeer used such an apparatus is generously considered by Steadman, but in the last analysis he is obliged to reaffirm his principal irrefragable argument. By meticulous measurements and reliable calculations, he has located the viewing aperture in the front vertical of such a camera and traced the trajectories of projection through it of the simulated motifs of six of Vermeer's most famous paintings - so that their images and their perimetric dimensions are re-achieved on the rear vertical.

Steadman describes the history of projected actuality from its early beginnings to the time of Vermeer and his associates, who included the microcopist Antonis van Leuwenhoek and the camera painter and rapscallion Johannes Torrentius. Then he continues, to the time when Canaletto and Bernardo Bellotto switch on the daylight of 18th-century reality for us; this time, very largely and gratifyingly, en plein air .

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Camera painters were ideally concerned with producing the precursor of colour photography in a pre-photographic age; in Vermeer's case, a rare and lineless quality of painting that, until now, has nowhere been fully defined by precise description or emphatic enough contrast with other genres.

Steadman convincingly establishes the presence of Vermeer's camera in the room where he painted, and also identifies which room in the house this was and the most probable location of the house in Delft. He examines the effect of projected imagery on the spatial calculations in the paintings and on the chiaroscuro in their synthesis. Vagaries of projection, of light, and of inverse and reverse imagery, proportion and balance are closely observed. For good measure, Steadman shows exactly where one can best discern the telltale presence of a camera, for instance in those secondary passages of painting that are rendered more mechanically, such as that big, flat, dark jigsaw of drape patterns in the anteroom's foreground in The Love Letter .

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Throughout, there is much reference to the 1952 monograph on Vermeer by Lawrence Gowing, which contains intimations of the artist's use of the camera obscura. Gowing's interpretive genius and mastery of semantics are well seen.

Steadman concludes that incidental experiment with the camera obscura by other artists (for example, Carel Fabritius, Jacob Ruisdael, Gabriel Metsu - and today, David Hockney) is not comparable with the absorbed scrutinies of Vermeer. Picking up on Gowing's conjectures regarding the mirror reflections in Vermeer's The Music Lesson , Steadman has given us the most important ever diagrammatic exposition of the hidden impedimenta in this work.

Critics and historians have not always correctly analysed the meanings in great paintings, too often coming up with wild and sometimes laughable interpretations. In Vermeer's case, he may have been an intentional joker with the critics (it may even be he grinning and enshadowed in The Procuress ). The inclusion of light spots (circles of halation on the camera's ground-glass screen) in The Milkmaid , for example, and in the View of Delft , may well be deliberate and mischievous: Vermeer saying, "This is how I did it and here's the proof."

Vermeer's Camera brings us much closer to the motivations and mysteries of Vermeer, "the Sphinx of Delft", and should become the seminal treatise on his work.

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Quentin Williams is an artist and art historian.