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10-31-2012, 04:16 PM   #1
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DNG embedded previews

A detailed investigation of previews in original and converted DNG files from K-30 gives interesting results.
Here are details for an example image, which was also converted with AdobeDNGConverter v7.2:
  • Original DNG: 16,218 kByte, 4928 x 3264 (4936 x 3272)
  • Original JPG: 7,063 kByte, 4928 x 3264
  • Converted DNG without preview: 14,345 kByte
  • Converted DNG medium preview: 15,261 kByte
  • Converted DNG full preview: 16,771 kByte
I extracted the previews with EXIF Tool v9.03 and the following parameters:
  1. exiftool -b -copy1:PreviewImage -w %%f_cp1.jpg -ext DNG -r .
  2. exiftool -b -PreviewImage -w %%f_pre.jpg -ext DNG -r .
  3. exiftool -b -JpgFromRaw -w %%f_raw.jpg -ext DNG -r .
The command "exiftool -b -ThumbNailImage" did not result in any JPG export. I also crosschecked with the special command "exiftool -b -BigImage". The results for exported JPG previews from above DNGs are as follows (The red numbers are results with above EXIF-Tool parameters):
  • Original DNG: 1. 1436 kByte 4928 x 3264, 2. 1476 kByte 4928 x 3264
  • Converted DNG without preview: 2. 40 kByte 640 x 480
  • Converted DNG medium preview: 1. 131 kByte 1024 x 678, 2. 170 kByte 1024 x 678
  • Converted DNG full preview: 1. 134 kByte 1024 x 678, 2. 174 kByte 1024 x 678, 3. 2292 kByte 4928 x 3264
The compression quality of original JPG and previews is quite different (98% and 34%, 49%, 82%). It seems also, that some JPGs use color subsampling and others not. Interesting is, that the converted "DNG without preview" includes data, which can be exported with EXIF-Tool as 640x480 JPG, but this data apparently is not used for DNG display from image viewers like IrfanView. Finally the converted DNG with full preview is larger than the original DNG, because the full resolution preview quality increased from 49% to 82% and its file size from 1436 to 2292 kByte. I know also, that there might be other preview data (TIF?) in the DNG files, which is used from specific software. But I do not know how to extract this data and if this is really relevant for usual image software.

Currently I am shooting always DNG+JPG, but I am not sure if the JPGs are really required. They are quite large, but on the other side the previews in the original DNG are smaller and have less quality. May be it is better to create new high quality JPGs after RAW processing and the original JPGs are useless and justing waisting SD card and disk space?

I also convert all DNGs to DNG with medium preview, because I do not like full resolution previews in DNG files. First I like to control, if other software is using previews or is really rendering the DNG. This is not possible with full resolution size previews. Second I like to keep the DNGs small, write protected and with XMP sidecars. This speeds up backup! So I have no intention to update previews or metadata in the DNGs.

Any comments about this confusing preview situation from your side?

Regards from Peter

Last edited by Plentax; 10-31-2012 at 04:37 PM.
10-31-2012, 05:12 PM   #2
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Posts: 4,616
The TIFF that you see is most likely the RAW data in the DNG, DNG writes the RAW data away as TIFF/EP and uses other standards to add more functions and features.

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