This article opens with a distinction between David Hockney's strong and weak theses. According to the strong thesis, in the period 1430-1860, optical tools (mirrors, lenses, the camera obscura, etc.) were used in the production of paintings; according to the weak thesis, mirrors and lenses merely inspired their naturalistic look. It will be argued that while for the fifteenth and sixteenth centuries, there is little evidence in favor of the strong thesis, the case is different for the seventeenth century, for which the use of optical instruments by painters is a documented fact. In this article, an early case is examined. The extant preparatory drawings of Gaspare Vanvitelli (Gaspar van Wittel, 1652-1736) suggest that this cityscape painter relied on a camera obscura. But even here, the strong thesis must be tempered. The fact that several stages of artistic transformation separate the camera obscura projection from the finished painting undermines Hockney's analogy between optically assisted painting and 'naturalistic' photography.