I'm not sure what it means but if I do a RAW conversion of a 5D file and a P30 file the P30 fills the histogram (16 bits) the 5D does not (12 bits)
1. The appearance of the histogram does not depend the very least on the bit depth.
2. I displayed both images with ACR; after WB on a grey square, resetting brightness and contrast to 0 and increasing the "exposure" of the P30 image by one EV, both histograms are filled.
Note, that ACR makes an automatic -1 EV adjustment to P30 and P45 images @ ISO 100, +1 @ ISO 400 and +2 @ ISO 800 (I don't know how this is @ ISO 200). I don't know the reason, but if you want to see the shot with the original exposure, you need to compensate for the automatic adjustment (it is not indicated anywhere by ACR!).
3. Several squares show clipping in both images.
Now, what about the raw data?
The first two attached images show the raw histograms.
The 5D image shows the perfect exposure. The exposure is maximized, but nothing clipped (apart a few stray pixels). Note the saturation level: 3692.
The green in the P30 images is significantly clipped: the white square (the bottom left) is completely clipped, *but nothing else*. The saturation level is 65535.
So, ACR's showing several squares as clipped is plainly incorrect - but why?
Let's see, what makes the ACR histogram sop different from the raw histogram?
*Note*: the following does not include
b. color transformation from the camera's color space in sRGB or aRGB.The missing steps would cause other differences as well.
The first step is white balancing, on the middle grey square. This yields the following coefficients:
5D: red = 2.04, blue = 1.35
P30: red = 2.48, blue = 1.29
Now, let's see the histograms after WB application and mapping ("gamma encoding").
The third image is of the 5D. It looks good, except that there is some red clipping (the rightmost bump comes from the ominous white square of the checker). This red ended originally somewhere at 1830; after WB application it became 3733, and that is over the current white point (which started out with the saturation level).
The solution is either decreasing the brightness, or increasing the white point. I chose the latter one; see the fifth image, showing the histogram with white point 3942 - and there is no clipping any more.
The same could be achieved in ACR by either reducing the "exposure", or, much better, increasing the "recovery" (which does not recover anything in this case, because there is nothing lost).
Now, to the P30 shot. The fourth image shows the histogram after WB application and mapping. The bump of the white square vanished completely, i.e. it is cipped; thisis due to the WB application. Again the red dictates the upper limit: it ends around 29000, that with the WB coefficient 2.48 yields 72000. After increasing the white point to 72000 the red and blue clipping vanishes (see the sixth attached image). However, the green remains clipped: that is truy lost.
Finally, one note to the P30: it has a (small) weekness in comparison of the 5D. The difference in response between the red and the green is much higher with the P30. This results in a somewhat reduced dynamic range *in this lighting*.