An Illuminating Question – Dynamic Range

A recent question to the askRodger section of the pictureline web site has prompted this answer.

A gentleman was attempting to photograph his daughter’s high school on stage dance recital.  There were a total of five dancers in the performance, positioned at various distances on the stage.  The first dancer, dressed in white, was at the left front stage, in full light.  A second dancer, wearing black, was at center stage, approximately four feet deeper than the first.  Dancers three, four and five, also wearing dark colors, were at stage right, by the rear curtain, approximately eight feet from the light.

The photograph was properly exposed for dancer number one, who’s clothing was white in tone with the slightest hint of texture.  The face and arms of dancer number two, by default, was considerably darker in tone and the costume was black without texture.  The other three dancers’ faces and arms appeared as disjointed limbs, barely visible in the murky blackness without separation of the costumes and curtains.

The question with regard to this situation, was which option should be used in order to obtain a correct exposure for the entire scene, a tripod, an image-stabilizing lens, or faster (higher iso) film?  The answer is none of the above.

The answer is really in two parts, because of separate problems occurring simultaneously.  The first has to do with the overall dynamic range of film and/or digital sensors.  There is approximately only a seven or eight f/stop range that the recording media (film or sensor) can capture in a single exposure.

The second problem has to do with the Law of Inverse Square, which means that light from a source changes in intensity proportional to the inverse of the square of the relative distance from the source.  In other words, doubling the distance will cut the level of intensity by a factor of four (2 stops) not just in half.  If the distance is quadrupled the intensity decreases by a factor of sixteen (4 stops).  It now becomes apparent that the intensity of the light needed for adequate exposure will dwindle to insufficiency, swiftly.

The scene breaks down like this:

In direct light a white costume with very slight texture would reflect about 88.5 percent of the light leaving it 12.5 percent black (very white) and typical Caucasian skin reflecting 62.5 percent would be at 37.5 percent black or light gray.

Four feet from the light the skin tone will reflect only 12.5 percent or four stops less light becoming 88.5 percent black (very dark).

Eight feet from the light source it would be near impossible to determine tone because the dynamic range of the film and/or sensor cannot contain that amount of excessive overall contrast.

It now becomes evident that neither a tripod, an image stabilized lens, or film change would be relevant factors in remedying this situation.  By understanding how dramatically light falls off, the solution becomes one of moving the main light source further away from the first subject lessening the ratio of distance.  Of course, the power or intensity of the flash unit (or other light source) must be increased to provide proper exposure from the new distance, as well as provide a small enough f/stop for sufficient depth of field (area of acceptably sharp focus) to cover the entire group.  The key is a flash with a lot of POP.

I hope that this helps clarify how to photograph groups of people without leaving the last row out in the dark.  The photographs at the following link are from Digitalfest 2004 (our latest extravaganza), they will show how a single flash can illuminate a very large area.