Old but interesting post. I wonder if the sun in Ray's linear histogram is really not blown in any channel. The histograms look blown to me (see that little peak right in the end of the histogram line), but this does not happens in the end of the histogram as level values seem to have been corrected afterwards by some scaling (probably WB).
This happens sometimes if a RAW developer with less than 1.0 multipliers for the WB is used (like DCRAW for instance).
Another issue: did you know that scaling from 12-bit RAW to 16-bit in DCRAW uses a slightly greater than 16.0 multiplier?. Maybe this happens only on Canon cameras, but Dave Coffin confirmed this to me as I saw 16-bit histograms with peaks not equally spaced in 16 levels but a bit more (~17 let's say). That means (at least Canon cameras) don't make use of the full 12-bit: 0..4095 range.
For practical reasons, most digital cameras do not make use of the full 12 bit (0..4095) range. The pixel of a digital sensor can be likened to a bucket (or well) that collects electrons. When the pixel has collected all the electrons it can hold, this is called [a href=\"http://www.photomet.com/library/library_encyclopedia/library_enc_fwcapacity.php]full well[/url]. The output of a CCD sensor is voltage, which is amplified and presented to the analog to digital converter (ADC). A 12 bit ADC has possible outputs of 0..4095. This output is in units of analog to digital units (ADUs) and this represents the raw pixel value. The amplification is chosen so that at the full well of the sensor, the output of the ADC is at or near its maximum as explained in the reference. In practice, it may be difficult to obtain an exact match, so a little leeway may be allowed and the output of the ADC may not quite 4095. In addition, the sensor response may not be linear near full well, and only the linear portion of the sensor may be mapped to the ADC output.
To test the range of the ADC that is used, one can make a series of exposures until the sensor is saturated and the ADU value no longer increases. DCRaw is often used to examine the contents of the raw file, but I have found that Iris
is more convenient, since it has a graphical interface and also has tools to analyze the resulting conversion. To use Iris, one loads the raw file and performs a demosaicing operation, which renders the raw file into an RGB 12 bit form. No white balance is applied, and one can examine the values in the raw channels by placing the cursor over the area of interest. If desired, a white balance can be applied with multipliers. The file can be converted to the Photoshop 15 bit format (0..32,768) by multiplying the channels by 8 or to a conventional 16 bit file by multiplying by 16.
With my Nikon D200 (which uses a CCD), the ADU output at saturation is about 4009, rather than 4095. In the case of Ray's sun disk, the sensor is most likely fully saturated and there is no "underexposure". Canon cameras use CMOS rather than CCD and the pixel outputs a pixel value directly than presenting a voltage to an ADC, but the principle is similar.