Strobist 102: Apparent Size

To a subject, the apparent size of the light source will be a large determinate of the quality of light. In order to demonstrate this to myself and for the 102 Control, I photographed a Bing cherry. Lighting was supplied by a hand-aimed 550EX at 50mm with a Sto-Fen Omnibounce affixed to the front.

Aperture and shutter speed remained constant, the ISO, flash-to-subject distance, and flash power changed--from 100 to 400, one inch to nearly six feet, and 1/128th to 1/1, respectively.

Approximation of apparent size is found via the Small Angle Formula:

s' = s / d

Where doubling the distance (d) halves the apparent size (s'). While this is largely an approximation, especially at close distances, it allows for a structured set of tests with clear results.



One will notice how the highlights go from large and diffuse to small and intense. An increased apparent parallelism in the strobe is seen by the subject, creating harder light, and more intense specular highlights. Also noteworthy are the sizes of the specular highlights after eight inches; they are very similar. Perhaps the Small Angle Formula achieves sufficient accuracy between eight and sixteen inches.

Learn to light at Strobist.