PHOTOGRAPHIC DYNAMIC RANGE
Looking at the SNR curves provided at DxOMark, a very powerful but not so popular information since it is only available for each particular camera out of the comparison tool ("Full SNR" option):
(http://www.guillermoluijk.com/misc/titanes_snr_dxomark.png)
I have (re)calculated the Dynamic range for the Canon 5DS R, Nikon D810 and Sony A7R II, adding the A7 II and the A7S with a more photographic criteria than the one used in DxOMark:
- They chose a SNR=0dB threshold to define Dynamic range, perfectly valid as an engineering criteria so as to make comparisons between sensors. But quite unreallistic in the real world for a photographer (no Nikon D810 owner can dream of capturing a 14,76 stops scene in a single shot)
- A much more useful criteria would be to set a SNR threshold of 12dB (4 times less noise). This is the value IMO that allows to say "this camera can capture scenes with this number of stops between the shadows and the highlights".
The following image represents at a pixel level how noisy SNR=0dB and SNR=12dB are:
(http://www.guillermoluijk.com/article/digitalp02/ruido_0db_12db.jpg)
Taking the SNR values from the DxOMark curves and normalizing them to an 8Mpx output (the same resolution as used in the "Print" plots of DxOMark) from each camera pixel count we get the following SNR curves at base ISO (the D810 has some advantage here for being the only camera with a ISO64 base ISO, lower than the ISO100 found on the other four cameras):
(http://www.guillermoluijk.com/misc/titanes_snr.png)
To calculate the Dynamic range we just need to count how many stops we can find between sensor saturation (0EV) and the 12dB crossing point. Zooming the former plot to be more accurate:
(http://www.guillermoluijk.com/misc/titanes_snr_zoom.png)
We have the following Photographic Dynamic Range table:
(http://www.guillermoluijk.com/misc/titanes_rd.png)
Differences of up to 0,5 stops can be considered negligible; from that point a real advantage arises. As usual the Canon gets the worst results, although it is not so far from the Sony's as the usual "2 stops", specially compared to the A7S (a high ISO device). The Nikon D810 is the clear winner with more than 2 stops over the Canon.
These figures perfectly match those offered by DxOMark ("Print" mode for 8Mpx normalization), just need to substract 2 stops to DxOMark's Dynamic range figures. The reason is that in the areas where Dynamic range is calculated read noise is dominant, and read noise remains constant with exposure for a given ISO. This means going from SNR=0dB (DxOMark) to SNR=12dB (suggested photographic criteria) is equivalent to increasing exposure by 2 stops:
(http://www.guillermoluijk.com/misc/titanes_rd_vs_dxomark.png)
IQ IN HDR
With the calculated curves we can answer the following question: how important is the Dynamic range of a sensor to produce high quality HDR images? (in this case I'm assuming HDR=blending a bracketed series of exposures).
The answer is that the sensor's Dynamic range is irrelevant to HDR since in creating the HDR composite we are never going to use the deep shadow areas, where Dynamic range is defined. Instead we are just using the last stops before saturation (let's assume the last 3 stops of those curves in an optimum 2-stop intervals bracketing).
To find out the camera that will produce the cleanest HDR outputs we must look at the curves near saturation. In this way the best of the 5 cameras would be the Sony A7R II, but the advantage will be none in practice for two reasons: the gap is not so big (about half a stop or less), and all cameras will produce a very high SNR in those areas anyway. A cheap compact camera has enough Dynamic range to produce high quality HDR output images in terms of noise.
Regards.