My guess, based on what raw files should be and on what I read (though the information is certainly confusing and contradictory) is that this is what happens.
1. With HTP activated the camera is actually underexposing by about one full stop and recording the resulting image data into a raw file unaltered.
2. The camera is also including a flag or similar that notes that the file was underexposed in this way, so that conversion software that recognizes this flag can apply a different conversion curve to the data.
3. Some conversion software (DPP for sure, apparently Adobe products, and perhaps others?) recognize this flag attached to the file and know to apply the HTP curve to bring up the luminosity of the image file except the brightest values, in line with the HTP curve. (This also explains why conversion software that doesn\'t know about HTP simply gives a plain raw file conversion, and it explains how we can get an unaltered conversion from Adobe products.)
What is the alternative? To think that the HTP curve is \"baked in\" to the file data rather than attached as flag/alert to conversion software and that all conversion software that doesn\'t know about HTP does know about the \"reverse curve\" required to compensate for it. That last point makes little sense — things just aren\'t typically done that way.
Until I (or someone else) can test and prove otherwise, my assumption will remain that the HTP data is attached to the raw file and that it does not alter the basic data in the file.
Dan, who is certainly prepared to be proven wrong (or right!) about this.
Nov 05, 2016 at 09:10 AM
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