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The ability of a digital sensor to capture information over the whole range from darkest areas to lightest is called its dynamic range. You take many kinds of photos in which an extended dynamic range would be useful. Perhaps you have people dressed in dark clothing standing against a snowy background, or a sunset picture with important detail in the foreground, or simply an image with important detail in the darkest shadow.

However, sensors have some difficulty capturing the full range of tones that may be present in an image. Tones that are too dark won’t provide enough photons to register in the sensor’s photosite “buckets,” producing clipped shadows, unless you specify a lower threshold or amplify the signal, increasing noise. Very light tones are likely to provide more photons than the bucket can hold, producing clipped highlights and overflowing to the adjacent photosites to generate blooming. Ideally, you want your sensor to be able to capture very subtle tonal gradations throughout the shadows, midtones, and highlight areas.

One way to do this is to give the photosites a larger surface area, which increases the volume of the bucket and allows collecting more photons. In fact, the jumbo photosites in larger dSLR sensors allow greater sensitivity (higher ISO settings), reduced noise, and an expanded dynamic range. For comparison purposes, the photosites on an 8MP non-SLR digital camera with a 2/3-inch CCD sensor measure 2.7 microns each. The larger sensors on a typical 6MP dSLR measure 7.8 microns—almost three times wider. You can see why a 6 megapixel dSLR might produce better images with lower noise than a non-SLR that has 2 million more pixels. The larger photosites tell it all.

Dynamic range can be described as a ratio that shows the relationship between the lightest image area a digital sensor can record and the darkest image area it can capture. The relationship is logarithmic, like the scales used to measure earthquakes, tornados, and other natural disasters. That is, dynamic range is expressed in density values, D, with a value of, say, 3.0 being ten times as large as 2.0. As with any ratio, there are two components used in the calculation, the lightest and darkest areas of the image that can be captured. In the photography world (which includes film; the importance of dynamic range is not limited to digital cameras), these components are commonly called Dmin (the minimum density, or brightest areas) and Dmax (the maximum density, or darkest areas).

Dynamic range comes into play when the analog signal is converted to digital form. As you probably know, digital images consist of the three color channels (red, green, and blue), each of which have, by the time we begin working with them in an image editor, tonal values ranging from 0 (black) to 255 (white). Those 256 values are each expressed as one 8-bit byte, and combining the three color channels (8 bits x 3) gives us the 24-bit, full-color image we’re most familiar with.

However, when your digital SLR converts the analog files to digital format to create its RAW image files, it can use more than 8 bits of information per color channel, usually 12 bits, 14 bits, or 16 bits. These extended range channels are usually converted down to 8 bits per channel when the RAW file is transferred to your image editor. The analog to digital converter circuitry itself has a dynamic range that provides an upper limit on the amount of information that can be converted. For example, with a theoretical 8-bit A/D converter, the darkest signal that can be represented is a value of 1, and the brightest has a value of 255. That ends up as the equivalent of a maximum possible dynamic range of 2.4, which is not especially impressive as things go.

On the other hand, a 10-bit A/D converter has 1,024 different tones per channel, and can produce a maximum dynamic range of 3.0; up the ante to 12 or 16 bits (and 4,094 or 65,535 tones) in the A/D conversion process, and the theoretical top dynamic ranges increases to values of D of 3.6 and 4.8, respectively. These figures assume that the analog to digital conversion circuitry operates perfectly and that there is no noise in the signal to contend with, so, as I said, those dynamic range figures are only theoretical. What you get is likely to be somewhat less. That’s why an 16-bit A/D converter, if your camera had one, would be more desirable than a 12-bit A/D converter.

Remember that the scale is logarithmic, so a dynamic range of 4.8 is many times larger than one of 3.6. The brightest tones aren’t particularly difficult to capture, as long as they aren’t too bright. The dark signals are much more difficult to grab because the weak signals can’t simply be boosted by amplifying them, as that increases both the signal as well as the background noise. All sensors produce some noise, and it varies by the amount of amplification used as well as other factors, such as the temperature of the sensor.(As sensors operate, they heat up, producing more noise.) So, the higher the dynamic range of a digital sensor, the more information you can capture from the darkest parts of a slide or negative. If you shoot low-light photos or images with wide variations in tonal values, make sure your dSLR has an A/D converter and dynamic range that can handle them. Unfortunately, specs alone won’t tell you; you’ll need to take some pictures under the conditions you’re concerned about and see if the camera is able to deliver.