The dual-pixel raw contains 2 raw data sets; we will be calling them main subframe (Frame 1 in RawDigger)...
... and auxiliary subframe (Frame 2 in RawDigger).
We will start by inspecting the main subframe. For this shot, it contains specular highlights that are blown out. Below is a screenshot with such a clipped area sampled - see the grey rectangle marking the area selected on the left brush. The red overlay over the specular highlight indicates the blown out pixels. The statistics for this selection is on the top panel of Raw Digger, outlined in purple.
All the channels reach their maximums: 15872 = (16383 - 511), where 511 is the black level for Red, Green, and Blue channels (the black levels are indicated in the lower left corner of the screen shot); while the second Green channel (G2) is reaching 15871 (because its black level is 512, higher than the others by 1 data number / DN).
The histogram for this selection indicates, as expected, that there is a peak in extreme highlights in all 4 channels.
This is all business as usual. All 4 channels reaching maximum prohibit any highlight reconstruction.
Now let's inspect the second, auxiliary subframe. The maximum values for the same selection are now lower and there is no Overexposure indication - no red overlay over the highlight. Also, the per channel maximums now differ from one another.
We can see the same on the histogram: no sharp peaks hitting the right wall. Thus, the auxiliary subframe is not hard-clipped.
The highlight are preserved in the auxiliary subframe and they are clipped in the main subframe. The full 14 bit range is used for both the main and auxiliary subframes, and there are no voids in the histogram that would indicate digital manipulation to fill the range.
In other words, effectively, the auxiliary subframe is "underexposed by 1 stop", compared to the main subframe. For example, sampling the gray WhiBal card in the shot, we can see that the values in the main subframe
are nearly 2x the values in the auxiliary subframe:
Also see the histogram for the sampled area of the main frame:
and the same area of the auxiliary frame:
Channel averages for the selection on the main subframe vs. the auxiliary subframe:
This confirms that the difference between the main and auxiliary subframes is nearly 2x, or 1 stop, and that the auxiliary subframe can be used for highlight recovery (again, an additional 1 stop of highlights is preserved in the auxiliary subframe while being clipped in the main subframe), effectively providing one more stop of headroom in the highlights; and the dual-pixel raw file for this camera contains 15 bits of raw data, if you consider main and auxiliary subframes together.
Here is how you can check that the main subframe of the dual-pixel raw is the same as a regular raw. Compare two shots in Fine mode - taking the main frame from Dual Pixel raw (the left part of the picture below - the dual pixel raw and its histogram) and the "regular" Fine raw (the right part of the picture below - the "regular" raw and its histogram).
As you can see, they are identical within the margin of error of the experiment.
Our sincerest gratitude to the Imaging Resource Team for their continuous work and making raw samples available to the community.
PS Low ISO raw files from other recent Canon camera models do not reach the maximum number, 16383, possible with 14-bit data; Canon 5D Mark IV does. So, the main subframe may be formed by adding the data numbers from both subpixels and clipping the result to 16383, while the auxiliary subframe contains the data for only one set of subpixels, and at base ISO it seems to be "full-well limited".
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