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When you take pictures in RAW, the camera stores a preview image (possibly jpeg) that it shown on the screen using white balance and other adjustments set in the camera.

Where is this information stored, is it stored within the RAW file itself? If so, does it add a lot of extra size to the RAW file?

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Most RAW files include a thumbnail for easy preview without having to process the image. It is not full resolution and is heavily compressed, so you don't need to really worry about the space consumption.

Without it, nothing that doesn't know how to process RAW files would be able to show what the file contains.

Ultimately, it is up to the camera if it uses the thumbnail itself or renders the RAW data using the stored settings for display on the camera though. Some camera models also support doing custom RAW processing right on camera.

I suppose you could presumably hack a raw file to include a different thumbnail if you wanted to test the exact behavior of your camera, at least provided that that much of the RAW file spec is publicly known.

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In general, it's a safe bet that this information is stored in the RAW file itself.

With Canon, the preview is 1/4 the size of the original and a fairly heavily compressed JPEG, so I wouldn't worry about size.

  • No, it is not always 1/4 the size of the original with Canon. At least Canon 500D uses full size JPEG files (4752x3168 pixels). Typically, the total size of the RAW file is approx. 20 MB, and the JPEG thumbnail takes approx. 2 MB of that, so it is roughly 10 % of extra size. (Reference: download Canon 500D raw files from e.g. photographyblog.com/reviews/canon_eos_500d_review/sample_images and use "dcraw -e" to extract the thumbnails.) – Jukka Suomela Dec 22 '14 at 1:44

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