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When I open a photo for which I'd like to resize the dpi in Affinity Photo, I go to Resize Document and I see the default dpi is 72. When I change the dpi to 150, the pixels adjust accordingly. However, when I export that photo, reimport to Affinity Photo, and go back to Resize Document, it shows dpi as 72 again. Why does it not show 150? Any insight would be greatly appreciated as I want to ensure these are best quality for printing. Thanks!

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Any insight would be greatly appreciated as I want to ensure these are best quality for printing. Thanks!

 

I doubt my insight will be very helpful but from what I can tell, the whole embedded Exif metadata thing is a nearly hopeless mess. See for example this section of the Wikipedia Exif article. Its first & last bullet points are probably the most significant:

 

Because the Exif metadata can be spread out anywhere in the file, it is extremely hard for software to change it without damaging it unless it can decode & encode every tag that might be present. So AP takes the safe but less than desirable route of either not modifying it at all (except for the optional dc:description comment metadata, which can contain anything up to 64 KB of data) or removing all of it completely.

 

Worse, as the last bullet point (& other sources like this one) mention, the Exif DPI tag of photos is meaningless! What we see in apps like AP or Apple's Preview app are (I think!) a DPI value derived from three Exif tiff tags (ResolutionUnit, XResolution, & YResolution). Since AP takes the safe route, that never gets changed (unless all the Exif metadata is deleted during export).

 

If that is not confusing enough, for exporting to the JPEG format, there is also the JFIF (JPEG File Interchange Format) standard to contend with, which is incompatible with Exif! From what I can tell from Preview app's "Inspector" window, AP actually does write updated JFIF resolution metadata data to a JPEG export file (in the JFIF APP0 marker segment as Density Unit, X Density, & Y Density, similar to the tiff Exif tags) if "Embed Metadata" is checked, but that is useless because apps use Exif metadata to derive a DPI value.

 

One more thing to consider that makes this even more confusing: Exif is only one of three major metadata standards in common use for photographs. There is also IPTC & Adobe's XMP. They are all interrelated in ways too complex to mention here (or for me to understand more than superficially). The best thing I can say about this for Affinity Photo is it will preserve all of it present in the file if "Embed Metadata" is checked (& I think remove all of it if it is not), but updating it probably isn't going to be a feature of the app for a long, long time.


Affinity Photo 1.6.7 & Affinity Designer 1.6.1; macOS High Sierra 10.13.6 iMac (27-inch, Late 2012); 2.9GHz i5 CPU; NVIDIA GeForce GTX 660M; 8GB RAM
Affinity Photo 1.6.11.85 & Affinity Designer 1.6..4.45 for iPad; 6th Generation iPad 32 GB; Apple Pencil; iOS 12.1.1

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