The Vermeer camera obscura hypothesis turned inside out: complexities of experimental research Paul J. C. van Laar, Erma Hermens, and Gregor J. M. Weber ABSTRACT Despite the significant amount of attention the Vermeer camera obscura hypothesis has re- ceived in both academic and popular media, the body of experimental research upon which these elab- orations continue to be based is very limited. Furthermore, these reconstruction set-ups pose significant limitations in historical accuracy, in turn producing results that often lack congruity with 17 th -century optics and painting technique. This paper aims to address this gap in our knowledge by focusing on 17 th -century accounts of the camera obscura, in art theoretical or art technological treatises as well as optical texts. This reveals that knowledge of the camera obscura was relatively widespread across various disciplines, making it likely that Johannes Vermeer was at least aware of its existence and perhaps saw demonstrations of the tool. Historic descriptions and illustrations demonstrate a recurring explicit reference to projecting outdoor scenes. Furthermore, the lens was often described as a small or simple spectacle lens. These characteristics are taken as the basis of a critical re-evaluation of published experimental research, highlighting the signif- icant limitations of their conclusions. Two preliminary reconstructions provide context to these limitations and reveal that while 17 th -century spectacle lenses can indeed produce an optically coherent image, the use of a camera obscura with such a small aperture in an interior setting gives disappointing results due to the low luminosity of the projection. Introduction Ever since Pennell (1891, 295) first proposed that Jo- hannes Vermeer (1632–1675) used an optical tool in his painting process, virtually any discussion of the famous Delft painter, in both academic and popular media, has attributed certain characteristics of his works to the use of the camera obscura. It is surprising, however, that despite the abundance of references to this idea, writ- ers continuously rely on a limited body of experimental research to support their theories. These reconstruction set-ups pose significant limitations in historical accu- racy, producing results that often lack congruity with 17 th -century optics and painting technique. There is, therefore, still a lack of understanding of how a 17 th -century camera obscura would fit into Ver- meer’s artistic process. This paper aims to address this gap in knowledge by examining 17 th -century accounts of the camera obscura and highlighting some limita- tions of published experimental research. First, we in- vestigate how well known the optical tool was across various disciplines in the 17 th -century Dutch republic, and whether Vermeer might have had knowledge of it. Then, properties of the camera obscura in the 17 th century will be extracted from historic descriptions and illustrations. This will serve as the basis for a critical re-evaluation of the most frequently referenced exper- imental set-ups. Two preliminary experimental trials will provide context for some of these criticisms. Fin- ally, we offer recommendations for future research. The context of 17 th -century optics Natural science and optics The field of optics underwent revolutionary change in the early decades of the 17 th century, in large part through the work of Johannes Kepler (1571–1630). He was the first to correctly identify the retina, instead of the lens, as the recipient of visual information. The lens, instead, simply focused incoming light to correspond- ing points on the retina (Smith 2004). Quite famously, 67