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:


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?

Edit 2:

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.

  • 2
    I would be posting this question to community.synology.com. My first instinct is that there is not some "smart loss-less compression" of 10:1 happening in the Synology app that the iPhone wouldn't do natively if it was possible.
    – osullic
    Nov 7, 2021 at 13:49
  • 1
    Can you just use file backup and not some photo specific application?
    – Eric S
    Nov 7, 2021 at 19:55
  • For some wicked definition of "backup". But I don't think the "DS Photo app" is designed to do a "backup".
    – xenoid
    Nov 7, 2021 at 20:32
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    A comment on most answers: I've often found that the smarter hardware claims to be, the more it can mess up. More basic tools don't have enough processing power to care about the contents of files, so they just do as they're told. I've seen this with portable card-reader hard drives (Canon did one it and only copied files Canon cared about - useless as a general backup tool). I've seen it with IP cameras (the cheap one could be over a LAN without going through their server, the more expensive one couldn't) etc. My cheap NAS drive just stores/serves files (downsides relating to power saving)
    – Chris H
    Nov 8, 2021 at 9:30
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    @slingeraap Please post your solution that you wrote as a comment, as an answer. That way, it can be voted on, and you can mark your answer as the accepted answer for the question. Thanks! =)
    – scottbb
    May 15, 2022 at 14:12

3 Answers 3


Does this imply that during the backup the contens of the DNG file have been converted from a TIFF format to 8-bits JPEG?

Yes. Personally at this point, I would regard the Synology system as not fit for purpose and find a different backup system.


Conversion of iPhone Images to jpg is a "feature" of Synology's DS Photo app.

Per the DS Photo page

To ensure optimal compatibility with Photo Station, DS photo will convert photos in HEIC format to JPEG format when uploading photos with devices running on iOS 11 or above.

To go further, DNG is a container that holds a TIFF and a bunch of metadata. A TIFF is another container that can hold a jpg.

A TIFF file, for example, can be a container holding JPEG (lossy) see here

From Synology's perspective, it probably makes business sense because the alternative would generate a lot of customer support work because the app is a consumer product and the average consumer would be confused at best and upset at worst.

You are probably best managing your pictures as files using the Synology file management app instead.

  • 1
    I found that note too, but that is about the HEIC format (which is what iPhone uses for non-RAW pictures). I haven't been able to find an official statement from Synology on ProRAW. Automatic conversion to JPEG doesn't seems logical to me, because that defeats the purpose of shooting in RAW.
    – slingeraap
    Nov 8, 2021 at 8:13
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    @slingeraap you're thinking like a photographer, not someone who wants to oversell storage. Recoding/recompressing is also dumb except at the final publishing step, especially as the HEIC you mention is in many ways better than JPEG
    – Chris H
    Nov 8, 2021 at 13:38
  • I marked this answer as the accepted one, because of the reference to the DS File app that also includes a Photo backup option, but one that only optionally converts files.
    – slingeraap
    Nov 8, 2021 at 14:47

Use Synology's DS File app instead of DS Photo to backup photos from iPhone to (Synology) NAS.

DS Photo does an unconditional and implicit conversion to jpeg. HEIC pictures are converted to jpeg. The same happens for ProRAW pictures, but the file is saved as a .DNG file that contains the jpeg. So although the file name suggests that we have a RAW file, in fact it is a plain jpeg file.

DS File also has a photo backup function. DS File has an option to enable/disable automatic conversion to jpeg. When this option is disabled, the file is transferred 1:1 to the NAS without any changes in quality or file type.

(Thanks to Bob Macaroni McStevens for pointing me towards DS File.)

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