"Significant" Values

The key terms used in describing light when metering are highlight, middle values and shadow. Highlights are the brightest parts of the scene; shadows are the darkest; and the middle values lie between.

Highlights can be broken down into two main types--the principal, or textural highlight, and the specular highlight. The principal highlight is the brightest part of the scene in which detail, or texture is to be recorded, while the specular highlight is just bright tone (brightness) with no detail or texture. It may "read" on the print as paper white (the brightness and color of the paper base), something you usually want to avoid with large areas within the frame.

A principal highlight might be a freshly-painted picket fence in which you want to show the grain of the wood, or an adobe wall which has been freshly plastered. This highlight needn't be bright white; it is just the brightest value you are recording with detail or texture in the frame. A specular highlight has no texture. It can be the glint of light off an afternoon lake, or the glare from a glass-and-steel skyscraper. There is no recording of detail in this tone--it is pure light, or bright white in the print. Having some specular highlights is great--in fact, it is what often gives prints "sparkle", and you can preview them being forced into the image by holding down the ALT/Option key as you pinch the Levels slider, for example.

The other end of the brightness scale is the shadow area. The key term here is "significant shadow detail", or the darkest part of the scene in which you want to record visual information. There may be darker parts of the scene, but these will record as tone (deep gray to black) without textural information or detail.

These terms, "principal" and "significant" are interchangeable, but are very important to keep in mind when making exposures. While we do have a wide recording range on film, and must make every effort to control highlight exposure with digital, seeing the image in terms of a tonal spread, and how that spread will record, is key to making good exposures.

For example, say you are photographing a white car in bright sunlight. You take a reading off the white hood and roof and get f/16; you read the shadow cast by the car and get f/4--this presents no exposure problem, as long as you bias the exposure towards the highlight and shoot at about f/11. However, you also take a reading from the tires on the car in the shadow area, and get a reading of f/2; this is clearly out of range and you might have to resort to some fancy shadow retrieval work to get any detail back, though it is often not worth the noise and bother.

The question then becomes, is it important to record detail in the tire tread, or can the tires record as pure black (with no visual information other than tone?) If the answer is that the tires may record as pure black, the significant shadow detail is the information in the cast shadow of the car. If no, then other steps must be taken to record both the bright car and the tire tread. These steps may include adding auxiliary light (fill flash); using a reflector card to bounce light into the shadow area; or exposing so that the tires become the significant shadow area, then compensating for the overexposure of the highlights by making another exposure and then combining the two later. Or, with film, it might mean reading the shadow area in which you want detail, dropping two stops and then underdeveloping by from 10-20%.

Photo and text copyright George Schaub 2010. Seeing and reading scenes for "significant" areas of highlight and shadow is the key to good exposure. In this still life the textural quality of the objects is defined by how the brightness values are placed on the exposure scale.