Histogram accuracy when shooting RAW

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When you take a RAW shot the camera also processes and embeds a JEPG into the RAW file itself. This JPEG (processed using your in camera settings for JPEG processing) is what is used to make the histogram and is displayed on the camera LCD. It's also the image you see when you open the RAW file in windows and other picture viewing applications before you process the RAW itself into a proper image file.