Because the measurement is scaled for spatial frequency (in the case of DxO, it's scaled from 36 MP to 8 MP, IIRC).Originally Posted by jrw
At very high levels of detail (i.e. 100% crop, or a 6-foot-wide print), the noise power is high, and so the dynamic range is less than 14 stops. But if you only look at low spatial frequencies (i.e. downsampled to 8 MP, or an 8x10 print), the noise power is lower, and so the dynamic range is higher. That is one reason why it's necessary to have an intermediate format (such as 16-bit tiff) that has higher bit depth than the original (e.g. 14-bit raw). Without it, operations such as downsampling would introduce quantization error such as posterization.
That said, it's still possible (but not desirable) to have more dynamic range than the bit depth. All it means is that the shadows will have quantization error, which manifests as artifacts such as posterization and noise. It would be better to consider those parts excluded from the dynamic range, though and in fact remove them through dithering. Fortunately, very few cameras have ever had that "problem" -- most have enough noise that they don't ever have to worry about having too much dynamic range for their bit depth.
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