Chapter 13: Exposure Times for Slow Photography

What we call slow photography utilizes exposure times of greater than one minute, even for bright outdoor scenes, because either the light enters through only a tiny pinhole, or a light detector of very-low sensitivity (VLS) is used. The separate cases of pinhole photography, VLS photography, and solargraphy (a combination of the two) are detailed and tables of typical exposure times are presented. Ordinary photography, using lenses and high-sensitivity detectors in combination with bright scenes, is described as a point of comparison in order to illustrate basic principles of photographic exposure. The mathematical relationship between the light illuminating a scene and the illuminance at the focal plane of a camera is described as a method for using a particular VLS photographic material as both an incidence light meter and the detector in a camera. A measure of outdoor scene brightness is defined, and ISO sensitivity of ordinary photographic detectors is contrasted with a “slowness,” defined as a more-practical measure of sensitivity for VLS detectors.