ByMark W Lucas
Ideally the photos we take with a digital SLR camera will duplicate what we see with our eye. This does not occur because of what is called dynamic range. The dynamic range of the camera determines how closely the photo will resemble what the human eye can see regarding color, contrast, shadows, and highlights. Cameras just cannot yet duplicate what the human eye can see.
This is not only a problem with digital cameras, but has also been a problem with film cameras. As digital SLR cameras evolve, the dynamic range of these cameras has been improving also. This means the pictures you take more accurately record what you see with your eyes.
All photos have shadows and highlights which are ultimately affected by the quality of light present for the photo. The shadows and highlights are more or less noticeable depending on the contrast of the scene. The contrast is affected by the amount of light present. High light levels, such as bright sunlight, and bright bulb flash result in high contrast. Low light levels such as an overcast day, muted indoor light, and diffused flash result in low contrast. The term hard light is used to refer to bright light (high contrast), and soft light refers to low-level light (low contrast).
When there is a high degree of contrast in a scene, the human eye sees details in both the shadows and the highlights. Digital SLR cameras cannot see the same detail in both the shadows and highlights. This is because it cannot detect and record the entire dynamic range of the scene. The dynamic range of the scene is the complete range of brightness from the darkest areas (shadows), to the brightest areas (highlights).
Digital SLR cameras are able to accurately capture the shadows or the highlights, but not both. When the camera captures the shadows well, the lighter areas of the scene become over exposed and appear too light and washed out. When the highlights are captured well, the darker areas of the scene become under exposed and appear too dark. Image editing software can restore the lost detail of shadows. The detail is actually there, and if you lighten the dark areas, the detail will appear. Unfortunately, when areas are too light and washed out, the detail is gone, and cannot be restored by darkening the area with software.
One solution to this problem is high dynamic photography. This is a very tricky and labor intensive technique involving blending several images of different exposures together to arrive at the final composite image. This is done with computer software and is difficult to pull off well.
Camera manufacturers are now building cameras with what is called extended dynamic range, or more accurately, enhanced dynamic range. It is not really possible yet to extend the dynamic range of a camera, so it is really just being enhanced.
There are two ways to accomplish this:
Brighten the highlights, but not so much that they lose detail and look washed out.Expose the scene in favor of the highlights, thus preventing them from becoming washed out. Also, the brightness of just the shadows is increased to more accurately balance the image.
Different manufacturers take different approaches to the problem. For example, Canon uses the first approach and calls it highlight tone priority. It monitors the highlights and preserves some amount of detail in those areas. To the contrary, Nikon has chosen to take the second approach, and calls it active D-lighting. It adjusts the exposure for the highlighted areas, and then at the time of exposure, increases the brightness of the shadow areas. Other camera manufacturers will have different names for similar dynamic range improvements.
We are still in the early stages of this technology, so just expect minor improvements in the dynamic range dilemma as it's development progresses.
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