Perceptual Bases for Rules of Thumb in Photography

Photographers utilize many rules of thumb for creating natural-looking pictures. The explanations for these guidelines are vague and probably incorrect. I will explore two common photographic rules and argue that they are understandable from a consideration of the perceptual mechanisms involved and peoples’ viewing habits.

The first rule of thumb concerns the lens focal length required to produce pictures that are not spatially distorted. Photography textbooks recommend choosing a focal length that is ~3/2 the film width. The textbooks state vaguely that the rule creates "a field of view that corresponds to that of normal vision" (Giancoli, 2000), "the same perspective as the human eye" (Alesse, 1989), or “approximates the impression human vision gives” (London et al., 2005). There are two phenomena related to this rule. One is perceived spatial distortions in wide-angle (short focal length) pictures. I argue that the perceived distortions are caused by the perceptual mechanisms people employ to take into account oblique viewing positions. I present some demonstrations that validate this explanation. The second phenomenon is perceived depth in pictures taken with different focal lengths. The textbooks argue that pictures taken with short focal lengths expand perceived depth and those taken with long focal lengths compress it. I argue that these effects are due to a combination of the viewing geometry and the way people typically look at pictures. I present demonstrations to validate this.

The second rule of thumb concerns the camera aperture and depth-of-field blur. Photography textbooks do not describe a quantitative rule and treat the magnitude of depth-of-field blur as arbitrary. I examine the geometry of apertures, lenses, and image formation. From that analysis, I argue that there is a natural relationship between depth-of-field blur and the 3D layout of the photographed scene. I present demonstrations that human viewers are sensitive to this relationship. In particular, depicted scenes are perceived differently depending on the relationship between blur and 3D layout.