Monday, October 8, 2007
Exposing For RAW
There are special considerations to take into account if you're shooting RAW and you want to be sure that you're getting a proper exposure
You wouldn't think changing image capture from film to digital photography would require a new way to think about exposure, but it may, depending on how you use your digital camera. This is because a digital camera sensor behaves quite differently from how film and our human visual system respond to light intensity. Digital cameras record data in a linear fashion. The human visual system responds to stimuli in a nonlinear fashion. Example: You enter a pitch-black room and turn on a 100-watt light; you see a fixed amount of light intensity, as would a digital camera. You then turn on a second 100-watt light, thus doubling the light output. Because humans have a built-in nonlinear visual system, the room doesn't appear twice as bright. Because of the linear behavior of a digital camera's image sensor, how-ever, the camera would record the scene as twice as bright.
What Is RAW Linear Data?
The digital sensors in our cameras translate the intensity of light energy (photons) into an electrical charge, which is converted to a digital value. In Figure 1, you can see how a hypothetical camera sensor can record six stops of dynamic range, from shadow to highlight, using 12-bit encoding (0 to 4096 levels). The darkest tone the sensor can record at this low light level has a digital value of 0. The brightest and largest number of light energy this sensor can capture has a value of 4096.
In linear capture, the first stop of highlight data contains half of all the levels within the image, and the next stop, half the remaining number of levels and so on. The fewest number of levels recorded is in the last stop of shadow detail—only 64 of 4096! If the image is underexposed, even fewer levels are used to describe that last stop of tonal data. This is also where most camera noise is found, so the results of underexposure are more noise, less actual data. Even with no exposure whatsoever, you'll find noise generated from ambient radiation. Shoot a frame with your lens cap on and then examine the RAW data in your converter, perhaps pulling a steep curve or moving the exposure sliders to examine the data represented in the dark shadows. You won't see all black, level-zero pixels, but instead some random noise.
Depending on the dynamic range of the scene being photographed, there may be darker tones the sensor can't detect and brighter tones that exceed what the sensor can record due to exposure (4097+ photons). In a scene that has 6½ stops of dynamic range, it's possible that a half stop of brightness can't be recorded because this portion of scene brightness occurs beyond full-sensor saturation. All light values above full saturation can't be recorded unless the exposure is adjusted. Doing so would result in a half-stop less shadow data being recorded. You must attempt to fit the dynamic range of the scene within the dynamic range the capture device can record.