I have a new iPhone which allows photos to be stored in RAW. I also use a Synology NAS and their DS Photo app to automatically copy photos taken on the iPhone to the NAS. I notice however that the file size of the files after copying is significantly reduced: from 20-something MB to somewhere between 1 and 2 MB. Both files are DNG files. If I compare the full size file with the reduced size, at first glance they look similar: amount of pixels is the same.
I would like to understand how the file is made smaller: is there some smart loss-less compression applied or has the picture's quality been reduced in the process? I cannot find any information online from Synology on how their process works.
Is there a tool that allows me to analyze the DNG files and that enables me to tell whether the original and copied files are functionally equivalent?
I opened both files in Photoshop and looked in the file info. I noticed that the reduced file contains the following tags:
<dc:format>image/jpeg</dc:format> <tiff:ImageWidth>4032</tiff:ImageWidth> <tiff:ImageLength>3024</tiff:ImageLength> <tiff:BitsPerSample> <rdf:Seq> <rdf:li>8</rdf:li> <rdf:li>8</rdf:li> <rdf:li>8</rdf:li> </rdf:Seq> </tiff:BitsPerSample>
whereas the original file contains the following tag:
Does this imply that during the backup the contens of the DNG file have been converted from a TIFF format to 8-bits JPEG?
On further inspection, it appears that the reduced file is in fact a JPEG file, although it still bears the .DNG extension. It starts with the bytes FF D8, which as far as I understand identifies the JPEG file format.