Expose Your Shots Properly
By Ken VanBree
Automatic exposure compensation has been one of the most important advances in photography in the last 30 years. When thru-the-lens built-in light meters were first introduced, they freed the photographer from the need for a separate light meter to properly expose each photograph. The first systems simply showed a needle in the viewfinder that moved up and down as the aperture and shutter speed were varied. To get the correct exposure, the photographer manually adjusted the settings until the meter needle was centered. These simple needle- matching systems gave way to more and more complex automatic metering systems that analyzed the exposure in multiple zones, and compared the exposure in each zone to a standard set by analyzing thousands of professional photographs.
Although the automatic exposure systems on today's digital cameras are extremely capable, they suffer from a limitation left over from the days of shooting film. The sensors in modern digital cameras cannot accurately record images that have a wide dynamic range, that is, a wide range of light values. If a scene contains objects that are in bright sunlight and objects that are in dark shadows you have a choice of exposing for the highlights or exposing for the shadows. The camera's digital sensor will record one or the other, but not both. This is illustrated in the figure nearby. In the left most image, the exposure was set to capture the highlights of the outdoor patio. The result is that all detail in the couch is lost in the shadows. Similarly, the middle exposure brings out the detail of the fabric in the couch and the pattern on the pillow, but the outdoor scene is completely overexposed. The issue is that the camera's sensor does not have enough dynamic range to expose all parts of the image properly.
Film photographers have dealt with this issue for years. Ansel Adams developed the Zone System to deal with this problem when shooting black and white film. By carefully controlling the exposure of the film and the film development time, black and white photographers are able to record images in which the light value differs by 10 f-stops. But color slide film and digital cameras can record less dynamic range, about seven f-stops. This means that if you are photographing a scene with widely varying light values, such as the inside of a wall around a window looking out on a brightly lit outdoor scene, you will have to choose which parts of the scene you want properly exposed. You will need to over- expose the objects outside the window in order that the walls inside the house will be properly exposed.
One trick the pros use to deal with high dynamic range scenes, is to capture two or more images of the scene and compose them together in the physical or digital darkroom. The right most image shown here was generated by taking the outside highlights from the image on the left and the shadow detail from the center image and combining them in Photoshop. The resulting image shows good detail over a wide dynamic range of light values that could not be captured in a single photograph. It helps to use a tripod to steady the camera between shots when you are doing this type of high dynamic range (HDR) photograph.
Automatic exposure compensation is a wonderful thing, but it can easily be fooled by scenes that have a wide variation in light intensity. In such cases, you may need to switch to manual control and set the overall exposure for the photograph using the portion of the scene that is of most interest to you.
August 2005 Builder Architect Edition Issue