Art and Optics : Susan Grundy: Camera-obscura

What is a camera obscura anyway?

	The above diagram graphically illustrates, in a simplified way, how Caravaggio might have used a "mirror lens" early in his career. The figure on the left is meant to represent a "live" model; on the right is the imagined projection. David Hockney claims that outside of a "sweet spot" of about thirty centimetres, it is impossible to get the image into focus. (Artwork author.)
	The above diagram graphically illustrates how Caravaggio might have used a lens when he first moved to the Palazzo Madama. The biconvex lens not only inverts the image but also flips it left-right. (Artwork author.)

Charles Falco, David Hockney’s partner and scientist working with optics, describes the camera obscura “as something to make, not something to buy.”¹⁸ He recommends working in a bathroom with a small window in which the right ambient level of lighting can easily be achieved. To enhance the camera obscura effect either a shaving mirror (concave) or a magnifying glass (a biconvex lens) can be utilized. If a shaving mirror is being used it should be aimed at the wall alongside the small window. An upside-down image will appear on the wall. With a magnifying glass the image must be reflected onto the wall opposite the small window. The image will not only be upside-down, but will also be reversed. The concave mirror is considered by Hockney and Falco to have been the first technical improvement on the natural camera obscura effect, while they consider the refractive biconvex lens to be the next step.¹⁹It is the concave mirror effect that Stork erroneously uses in his rebuttal rather than the later development of the biconvex lens.

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