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3. Quantifying Exposure The assessment of exposure, coined "exposure control" or "exposure compensation", is typically performed via a process known as metering. Light metering devices, such as the ones on your camera, measure the amount of reflected light reaching the camera from a given scene. We will discus “incident” light metering at another time, which is a measurement of the light falling directly on the scene, rather than the light which the scene reflects. Your digital SLR will depict the Exposure Value (EV) determined by the camera on your screen and in your viewfinder as a graph similar to the one below. The center arrow represents the camera's concept of correct exposure for the light in the scene, keeping in mind that this reading is relative to the metering methodology used by your camera at that time (some of these methods will be discussed below). The rectangular figure below the the 1 represents how you current exposure settings related to the camera's idea of correct exposure. The numbers to the left of the arrow represent 1 and 2 "stops" down in exposure value. The numbers to the right of the arrow indicated 1 and 2 "stops up in exposure value.
In the example above, the exposure reading is tell us that we are one stop under-exposed. We would want to adjust our exposure settings until the rectangle is beneath the arrow, reflected what the camera perceives to be correct exposure for the scene. We'll discuss this further in the following article. Since the advent of exposure meters on cameras, much progression has been made. The earliest, and simplest form of camera exposure meters took the entire scene, as seen through the lens, and averaged the intensity of the light across it to determine the necessary exposure. This recalls one of our troublesome examples from the previous section. When my girlfriend is standing in front of a brightly lit background, the average light in the scene is extreme, but being backlit, not nearly as much light is falling on her face. The average exposure determined by the camera assumes that much more light is falling on her than truly is; therefore, she is too dark (under-exposed) while the background is reasonably exposed. This may have led to the advent of center-weighted exposure metering. These light meters average the scene as well, but they put significant weight on the average for the center of the scene. This would likely help the scenario above, as my subject is in the middle of the scene, the camera weights the light reflected from the subject more heavily than the bright background behind to yield a more appropriate exposure of my subject. This works fairly well; however, what if the subject we want to be perfectly exposed is not in the center of our scene. We are back where we started. This adversity was somewhat overcome by the advent of more elaborate exposure systems known as evaluative or matrix meters. These sophisticated metering systems break the scene down into a complex honeycomb of “zones”, which software algorithms on the camera then analyze, weight, and use to determine an average exposure. These systems are extremely good at rendering consistently accurate exposures across a wide variety of interesting lighting situations. They do fairly well with the scene we’ve described above. However, any metering method or algorithm may only ascertain what is perceived to be “accurate” exposure. And there would obviously be some debate as to what constitutes “accurate” exposure. To the product photographer, this may mean the correct representation of color. To the artist, this may mean an exposure which renders an ethereal, surreal, or slightly exaggerated depiction of the scene in order to evoke some emotion or vivid recollection. That being said, wouldn’t it be wonderful to control our own exposure destinies? To utilize our metering tools as a reference by which we can tell the camera how the light in a scene should be painted? This is were the manual exposure control and the spot meter (found on most all prosumer cameras) are of irrevocable value to the photographer. <<PREV 1 2 3 4 5 6 7 8 9 10 11 12 13 NEXT>>
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