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High dynamic range (HDR) processing is a technique achieved using software that takes the best tones from several exposures and combines them in one HDR image. The dynamic range is the range of brightness levels in a recorded scene from the darkest shadows to the brightest highlights. This is typically measured in f/stops and has always been a problem for film users shooting on colour transparency film, especially on films such as Fuji Velvia. The dynamic range is around six f/stops from the brightest to darkest points. And while colour negative and black & white film users have always had a few stops to play with, even at a maximum of around nine stops most photographic systems struggle with high contrasts scenes. Several software programs offer the possibility of combining differently exposed shots to one high dynamic range (HDR) image. Also see "High Dynamic Range explained technique".
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Often quoted on scanner specifications to indicate the maximum tone range that the scanner can capture. A higher figure indicates that the scanner will capture more detail from the highlights to shadows. With flatbed scanners that are being used to scan prints this isn't too important because the tonal range has already been reduced in the printing process. A higher dynamic range is essential on film scanners when scanning transparencies that have a wide tonal range. Specialist films, such as Fuji Velvia, are said to have a range of around 3.7D, so the scanner needs to have at least the same dynamic range to ensure every bit of highlight and shadow detail is captured in one scan. Also see "High Dynamic Range"