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7. Interpreting Exposure The advent of digital photography has lent itself to a fascinating tool known as the histogram. This tool, used accordingly, can give you some instant feedback on the picture you’ve just taken. This comes in handy when your LCD preview is not easily interpretable due to lighting conditions or LCD quality. The histogram is a simple graph. The horizontal axis represents the entire range of brightness levels from 0 (pure black – shadows) to 255 (pure white – highlights). The vertical axis represents the number of pixels in the scene to fall within the given brightness level. So pixels falling to the far left are moving toward pure black (under-exposure), and those that fall to the right are moving toward pure white (over-exposure). This helps to indicate how much of our scene will be under-exposed, over-exposed, or just right. After all, as discussed above, photo-sensitive mediums have only a certain tolerance to variations in light. Beyond that tolerance detail is lost or greatly diminished. The goal of properly exposing your photos is to make as much of the scene, or at least the most important elements of your scene fall within the tolerance of the photo-sensitive element recording it. For example, lets examine the histogram of two pictures. First, below we see a misty picture of several cats. The mist greatly diffuses the light falling on grey cats of fairly neutral density and green grass of fairly neutral density.
The next picture we'll examine is intentionally high in contrast. In an attempt to aggrandize the setting sun over a fishing jetty, the picture was exposed for the vibrant lights the setting sun. This means that the sky is well exposed, but the shadowy jetty is almost in silhouette intentionally.
The histogram here shows what we'd expect as well. The far left of the histogram reflects those pixels that are at 0 reflectance - completely dark. As we would expect from the picture, we have some very dark, shadowy areas in this picture. The far right of the histogram reflects those pixels that are at 255 reflectance- completely white. As expected from the picture, we see a spike here as many of our pixels situated at the center and around the center of the setting sun are intentionally over-exposed. Finally, as expected again, there is a wide variation across the breadth of the histogram as the tips of the shadowy waves are illuminated slightly by the setting sun to aid the light sky in balancing the dark sea. Because much of this histogram is in the middle of the exposure, it would indicate to the photographer that this picture will have many options in post-processing, only losing detail in those pixels that fall too far to the left (shadow) or right (highlight). At this point I'd like to mention that the colorful histogram above is not typical of what you'll see on most camera LCDs. Its a powerful histogram generated by Adobe Lightroom, a wonderful software package that we'll discuss in later articles.
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