Electronics engineers define dynamic range as the sensor's full well capacity divided by the read noise (both expressed in electrons). In practical photography, one is interested in the amount of shadow noise that can be tolerated and posterization from an insufficient bit depth. At higher ISO, DR is usually limited by noise rather than posterization.
That's true at all ISOs with current cameras. Only the Pentax K10D comes close to having quantized RAW capture, and then only in the very deepest shadows at ISO 100.
Norman Koren[/url] discusses bit depth and dynamic range on his web site. If you require 8 levels in the darkest f/stop to avoid posterization (banding), then a 14 bit ADC would give 11 f/stops of dynamic range and a 12 bit ADC two stops less.
That way of looking at things doesn't apply when the noise is much higher than the ADU steps. It's all dithered noise. There is no clear posterization in the RAW data in any DSLR you can buy. You see it in *CONVERSIONS* because the conversions posterize the output. That's why you see colored blotches in blue sky; in an attempt to smooth away pixel-level noise, the converters smooth things out so that the red values posterize in multi-pixel blotches.
Norman's excellent Imatest program can determine dynamic range for a given level of quality (limited by shadow noise). Here are the results for a Nikon D200 at ISO 100. For the highest quality, one gets only 6.5 stops of DR.
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That is measuring the TIFF, however. It is not a spec of the sensor, or even a measurement of RAW data.
The sensor itself, at a very minimum, would be capable of at least 4 stops more DR than what is available at ISO 1600, for a camera that starts at ISO 100.
