Extracting Preview JPEG from DNGs that don't have a Preview JPEG

[jrausch]

I created some test DNGs using the Adobe DNG Converter (v16). Some had Preview JPEGs embedded, others did not.

When running ExifTool against these files I did get preview JPEGs out for SOME of the DNGs that did not have a preview JPEG embedded by the converter. Specifically, DNGs using Lossey Compression would output a very low res JPEG (far lower res that the JPEGs it output from the DNGs that DID have preview JPEGs embedded). For uncompressed DNGs without a preview JPEG Exiftool did not output a JPEG.

I thought running the tool with the options below would simply extract the preview image and would therefor not output a JPEG for the DNGs that didn't have one, anyone know why I am getting one for compressed DNGs that don't have one?

The specific command I ran was:
exiftool -b -PreviewImage -w .jpg *.dng   

[StarGeek]

There are other types of embedded images other than PreviewImage.  Use this command to see a list of all of them
exiftool -G1 -a -s -Preview:All /path/to/files/

You can use the this example command to extract any preview images to separate files.