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RAF files from older 16-megapixel X-trans cameras are about 30MB. Compressed RAF files from newer 24-megapixel cameras are about 20MB. Given the size savings, it would be nice if uncompressed files could be converted into the compressed format. How can this be done?

Compression using conventional file compression algorithms cuts the file size approximately in half. However, such files cannot be previewed or processed without decompression.

File-system level compression is not helpful because it is not supported by all file systems. It also still suffers from effects of large file sizes, such as reduced transmission speeds.

Although DNG supports compression, the X-trans sensor layout and color balance are not well preserved after conversion to DNG. DNG allows the original raw to be embedded to work around such issues, but doing so eliminates any potential compression size savings.

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I'm afraid that uncompressed-to-compressed RAW conversion isn't possible.
RAW files are specific to a particular camera model and if the model doesn't support that newer compressed format, the RAW file wouldn't be readable in the camera and maybe also in RAW development programs.
Anyway, camera manufactures don't offer this option.

Three work around options come to my mind:

  1. Conventional file compression + list of lower resolution lower quality JPEGs.
    If you compress 30MB RAW to 15MB (as you mentioned) and add an 1MB preview JPEG, the savings are still very good and that JPEG should be more than enough for a preview.
  2. Conventional file compression + software than can view RAW image files in an archive.
    FastStone MaxView

...even lets you view images in password-protected ZIP, RAR and 7-Zip archive files directly and instantly...
Digital camera RAW formats support, including CRW, CR2, NEF, PEF, RAF, MRW, ORF and DNG.

  1. Do you really need RAW files or are you mainly interested in the better bit depth? You can convert RAW files to a lossy format supporting 48bpp after developing them in your favourite development software and keep the ability of their re-processing (exposure, HDR tone mapping etc.) in the future.
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