Most mid- and high-end DLSRs offer two or three sizes for RAW capture. When the camera is generating the medium or small sized RAW files, how does it make them smaller? Does it capture less information onto the sensor? Does it capture the full amount of information and then apply some sort of in-camera compression? Does it do something else that I'm not describing?
Answer
Douglas Kerr gives a masterful and largely non-mathematical summary at The Canon sRaw and mRaw Output Formats . The situation is complicated and not perfectly understood, but much has been deduced by reverse engineering. Evidently sRaw is a 2 x 2 aggregation but with some chrominance subsampling; mRaw is likely a bona fide resampling (involving local interpolation), with heavier chrominance subsampling. One might indeed characterize each as a form of "in-camera compression" performed in a sophisticated way to optimize the appearance of detail to the human eye for a given output file size.
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