http://www.bbc.co.uk/history/british...amera_04.shtml
What happens is that the lens spreads bright points of light into so-called circles of confusion. Such effects would not be seen by the naked eye. The art historian Charles Seymour and the photographer Henry Beville have tried to reproduce these effects, by photographing a similar decorative lion's head in a slightly unfocussed plate camera, with some convincing success. Several authors, including Seymour, have argued that Vermeer painted this and another tiny portrait, Girl with a Flute, using a box camera. The sizes of both pictures are of the order of size of a typical box camera's screen.