westin* (Stephen H. Westin) writes:
I think he’s talking about converting 12-bit (per channel) RAW data directly into 16-bit (per channel) data and not 8-bit data.
So was I, hence the phrase "an image converted to 16 bits". I should have said "a raw image converted to 16 bits". By point is this: if you take the same raw file and conver twice in CS, once in 16-bit mode and once in 8-bit mode, the "Levels" histograms will be essentially identical on the screen. That is to say, the same raw value that is converted to 255 in the 8-bit image will be converted to 32,768 in the 16-bit image; any raw value greater than that will be clamped to 32,768 (or 255). You don’t gain any headroom simply by using 16 bits.
True, but the "headroom" is in the camera, not PS CS. By shooting RAW, you gain a little headroom in the camera, to (hopefully) avoid blown highlights. If you can avoid blowing the highlights in the RAW data (the camera’s LCD histogram may still say that the highlights are blown, though), then none of the values will be converted to 255/32768 in PS CS.
Exactly what I’m trying (unsuccessfully) to say: the default conversion (and Photoshop’s default conversion is, I’m sure designed to mimic what Canon does in this respect) will lose some values. The point of those sliders is to let you intervene in the process and recover this information, which can’t be recovered after the conversion.
[ I don’t have PS CS, so take the following with the appropriate chunk of salt. ]
While this may be true of the PS CS converter, others seem to be using a different approach to RAW & PS CS: they do not use the RAW converter in PS CS. Instead, they use dcraw (which is free, third-party software — not Canon’s) to do a plain linear conversion (no brightness or gamma correction) of the RAW data to 48-bit PSD data, and then read the PSD file into PS CS.
Note that, even though the *camera* may say that the highlights are blown, they may only be blown in the camera’s LCD jpeg preview, and may not be blown in the RAW data. Because of this, there is no clipping (by dcraw — I have no idea what PS CS does) or loss of information during the dcraw conversion process. This is because, assuming that nothing in the RAW data is blown, all of the RAW data are below the maximum value (and because no gamma or brightness correction is done).
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Darryl Okahata
DISCLAIMER: this message is the author’s personal opinion and does not constitute the support, opinion, or policy of Agilent Technologies, or of the little green men that have been following him all day.