Digital cameras see light very differently than what the human eye experiences. The eye is a marvelous piece of biomachinery that allows us to see and adapt to a large range of light intensity. While an extraordinary piece of machinery in its own right, a digital sensor cannot match the eye’s capabilities. Until technology catches up to millions of years of evolutionary development and refinement, knowing how a sensor behaves and how to manipulate the data it captures will help a digital photographer achieve the highest quality photographs.
During the exposure of an image, millions of photosensitive detectors act like tiny light wells and collect the photons hitting an image sensor. When processed by the camera or a computer, the photons captured at each photosensor site are counted and translated into an image pixel with a corresponding luminance value. Unlike our eyes, a digital sensor operates in a linear fashion. A completely empty photon well equates to black. One that has collected all the photons it can hold is pure white. If a typical digital sensor has a dynamic range of six stops (a stop is a doubling of the brightness of light) and can collect 4,096 photons per light well, the first stop, or the extreme shadows is represented by only 64 photons. The second stop has double the number or 128 photons. The third, fourth and fifth stops have 256, 512 and 1,024 photons of light levels, respectively, representing the change in luminosity. The sixth stop or the highlight tones of a scene are doubled yet again and are represented by 2,048 photons. That means that half of the luminance data from a photocell is used to describe only the brightest parts of a scene (See Fig. 1).
FIG 1. – LINEAR CAPTURE
Now, if our eyes worked in this fashion we would very easily be overloaded and completely blinded by exponentially increasing light levels. But this is not the case. Our eyes have a built in compression mechanism that enables us to see up to 20 stops of light, far more than the present capabilities of today’s digital cameras. Furthermore our eyes remap the linear nature of light to a non-linear tonal space where the gradient changes are much more uniform. Raw converters must do a similar thing and have the complex task of converting the linear capture into a gamma corrected space that more closely resembles human perception (See Fig. 2).
FIG 2. – GAMMA CORRECTED CAPTURE
Correct exposure of digital images is critically important to maintaining optimum quality of an image. Contrary to what our eyes tell us, Fig. 3 is actually a properly exposed digital file. Earlier in this article I mentioned that by virtue of the nature of linear capture, most of the information gathered by the sensor is used to describe the bright areas of a scene and the shadow portions are represented by very few photons or data. To get the best quality image possible, we want to harness this multitude of information and remap it into the mid and shadow tones. This is achieved by overexposing images just to the point where highlight detail is beginning to get blown out (notice the white, featureless sky and the luminosity data running off the right side of the histogram).
FIG 3. – OPTIMUM EXPOSURE
When you compensate for the overexposure at capture by reducing the exposure in Adobe Camera Raw, you remap much of the highlight information into the mid and shadow tones (Fig. 4). This creates a photo with much more information in the darker areas of the photograph and allows for increased editing latitude and further refinement. Poor exposure, especially underexposure, can result in the remapping and stretching of photon-starved shadow data instead of photon-rich highlight data. If stretched too far, this can result in the unwanted appearance of noise artifacts and banding.
FIG 4. – RAW CONVERTER EXPOSURE COMPENSATION
Recognizing the differences between the human eye and digital camera sensors is the first step to understanding how to get the most out of your images. Knowing how a photosensor records light and how software processes and manipulates image data enables us to compensate for the limitations in current digital technology and allows us to use our cameras to their full potential.