Why does a Canon CR2 raw file contain a small uncompressed TIFF preview?
Asked 11/17/2015
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I inspected Canon 5D Mark III CR2 files and found multiple embedded previews, including a small TIFF image in addition to JPEG previews. The TIFF preview is relatively large in file size for its pixel dimensions, appears low in brightness and contrast, and looks very different from the rendered raw image. What is this TIFF used for, and why would Canon store an uncompressed TIFF inside a CR2 file?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
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According to this document under section 2.6 IFD #2 it's the uncompressed small version of the picture which has no white balance correction applied.
CR2 files are based on the TIFF file format so it would make sense for the uncompressed data to be in the TIFF format.
Its purpose is mostly for displaying the RAW histogram in tools like Digital Photo Professional based on the uncompressed image data.
The TIFF preview also appears to be rendered linearly, with no gamma correction applied, and the white and black points set at the extremes of the sensor's dynamic range.
Originally by user25272. Source · Licensed CC BY-SA 4.0
user25272
10y ago
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The embedded TIFF is there as an uncompressed, small rendered version of the image data, typically without white balance and with linear tonal mapping rather than a normal gamma-corrected JPEG look.
Because CR2 is based on the TIFF container format, storing this preview as TIFF is natural. Its main use is for software that needs quick access to image data for analysis rather than for final display. In Canon software such as Digital Photo Professional, it can be used to generate the raw-style histogram from minimally processed image data.
That also explains why it looks dark and flat compared with the raw image you see after normal rendering: it may have no white balance correction, no gamma correction, and black/white points set close to the sensor’s full dynamic range. So while it seems wasteful, it serves a technical preview/measurement role rather than a visually pleasing one.
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