So, while I understand what your trying to say...I think it's a misnomer. Dynamic range is itself defining what's usable...so saying that you can only use part of what's usable... Redundant and inaccurate. And, incorrect. ;P
Ok, thanks for explaining, I'm really not much of a tech geek even after all this time on CR :-p ... so for my education and to avoid further confusion: With which word(s) would you label the dynamic range starting from the shadows when no banding is visible anymore? Because that's what I called "usable" as fpn is what makes your shot "unusable".
That is dynamic range. The definition, in mathematical terms, is 20 * log(MaxSignal/NoiseRMS), to get dynamic range in decibels. The NoiseRMS is the Root Mean Square of the noise...basically if you sampled the actual noise in each pixel, and computed the RMS from that, that would be your noise floor. Dynamic range describes the usable potential signal range from the noise floor to the maximum signal (saturation point, in the case of ISO, that is also the Full Well Capacity, FWC.)
Maximum signal strength defines the range from "zero to"...if you had no noise, you would have the full range, say from 0.000...1e- to 76606e- (in the case of the 6D). If you could use the full potential signal range, the formula above becomes 20 * log(MaxSignal/0)...but you can't do that, it's divide by zero, which is undefined (undefined because the value becomes infinite...but infinity is also technically undefined). So, if you had no noise, you would have infinite dynamic range. Because noise exists, dynamic range must therefor be something less than infinity. You can keep lowering the noise floor. In astrophotography, we use supercooled CCD cameras that have extremely low dark current (while a DSLR may have as much as 20e-/s/px, a cooled CCD can have as little as 0.002e-/s/px), extremely high Q.E. (77% with Sony's new ICX CCDs is not uncommon, and some of the older Kodak KAF sensors (now owned by TrueSenseImaging, TSI) had as much as 90% Q.E.), and very low read noise at optimal gain. Further, we use more extreme noise reduction techniques, such as bias and dark calibration, which eliminates fixed sensor pattern noise as well as hot pixel noise, leaving us with what is effectively a pure gaussian read noise and photon shot noise signal. We then average together dozens of individual light frames to reduce noise even more (reduction is SQRT(subCount), so if we stack 100 subs, we get 1/10th the noise). In astrophotography, dynamic range of a final integration can be 25-30 stops or more (and we do "stretching", basically shadow lifting, that is so extreme it would make the D800 cry at how utterly sucky it is!
). But it's still not infinite DR, because there is still noise. There is always noise.