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Are we living in a 72dpi world?  

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Have a friend who comes entirely from the "print world". She claims the photos she "used to send" from her old phone were 300 dpi, but now they are only 72 dpi, and the printer to whom she is sending, is complaining that they aren't high enough quality.  I'm just trying to help with a reasonable explanation that has eluded me and my Google fu. 

Her iPhone is a 7.  Not sure what her previous iPhone was, but obviously had a lower resolution camera, yet the old iPhone seemed to produce the photos that the print-person wanted and her "new" iPhone 7 doesn't. 

I made sure she was sending "actual size" photos from her phone (she was) and that her iPhone wasn't uploading to iCloud Photo Library and downloading the optimized version.  (she's still on the free 5GB plan, so no iCloud Photo for her). It doesn't seem like process of e-mailing is "downsizing" the photos, but perhaps somehow it is.

I imported her photos into Photos to gather more info. Sure enough, if you export any photo, it's 72dpi on hers (as well as mine), so apparently that's the way it is now in this "on-screen world".  I took a photo into Photoshop and changed the resolution 300 dpi, and sent it to the print-person but have not yet heard if she "approved" it.

So, how do I reconcile this "mismatch" in the resolution that the iPhone is taking, and that the print-person wants? Does each photo need to be "up-resolved" for printing purposes?   is it possible that an older iPhone was taking (or sending) photos at "300 dpi"?

Just trying to reconcile these opposed facts, both for myself and friend.

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2 Answers
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DPI is not actually an attribute of a photo, it is an instruction to a printer or a monitor on how to show the available pixels.

I believe embedded attributes that say "72dpi" are probably there for historical/compatibility reasons. Software that "obeys" the 72dpi instruction on an uncompressed iPhone photo would show it as 56x42 inches. Hence almost all software uses its knowledge of the hardware it is running on to match the available pixels in the photo to the available pixel resolution of the device.

My ref: https://forums.macrumors.com/threads/dpi-of-iphone-photos.2122402/  

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I am seeing the same in iOS 13 on my 2016 iPhone SE. I don't know whether the default "instruction" in previous versions of iOS was higher dpi in exchange for lower resolution.

Using only an iPhone to post-process a photo before sending it might be more difficult or just require a specific app. I would use Preview on my Mac to change the dpi to 300 and reduce the dimensions by at least 4x.

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