Photoshop displays image DPI as a different number than Preview DPI on a MAC

[Tom5E74]

I have to deliver a number of photos to a client at 300dpi. I resized all the images in the photo to 300 DPI and they all look great, all are very high pixel dimensions. When I highlight them in the finder and open them in preview as well, some of the images display a different DPI - some as low as 72. I opened up the images again in photoshop and all are 300 there. Why is Preview and Finder displaying a different DPI than Photoshop? The client is going to see this and seriously question my ability to follow directions. Thanks for your help.

[Bob_Hallam]

Photoshop is correct in this case.  So open the files in PS to check that the pixel dimensions and dpi are correct.  If so, no worries. 

[Tom5E74]

Thank you!

Is there any way to have the numbers match? The client doesn't have photoshop, I really don't want any kickback from them if they check the DPI in finder.

[Stephen_A_Marsh]

What file formats? There may be inconsistencies in resolution metadata. Using ExifTool or providing a sample image would help to answer.

[Tom5E74]

Thank you, this image attached here is displaying at 72x72 resolution and 300 dpi in photoshop. Is there a difference between resolution and DPI?

[Stephen_A_Marsh]

As I first suspected: "There may be inconsistencies in resolution metadata".

In reality, there is no such thing as a "300 ppi" PNG image (BTW, for a file it's pixels per inch, not dots per inch). I won't even open the can of worms fallacy that "print requires 300 ppi resolution"...

PNG is not a format designed with print output in mind. The PNG specification works in a unit value of metres, not inches or CM. I believe that Photoshop "translates" the embedded PNG-pHYs metadata into the closest PPI when opening the file.

This is what ExifTool reports in your sample file:

[PNG-pHYs] PixelsPerUnitX : 11811
[PNG-pHYs] PixelsPerUnitY : 11811
[PNG-pHYs] PixelUnits : meters

[XMP-tiff] XResolution : 72
[XMP-tiff] YResolution : 72

[XMP-tiff] ResolutionUnit : inches

So, 11811 (ppm) ÷ 100 = 118.11 (ppcm) x 2.54 (metric to inches) = 299.9994 which is 300 ppi rounded up.

Notice the XMP-tiff resolution value of 72ppi and unit of inches. This is what Apple Preview is (incorrectly) using to report the pixel density, overriding the PNG-pHYs data that Photoshop is respecting. Remove or change the TIFF metadata and the Preview app info matches Photoshop.

How did this incorrect and conflicting metadata get in there in the first place? Did you save as or use save for web or export to png?

So, what is the answer?

* Accept these facts and communicate/educate your client

* Use software such as the command line interface ExifTool to remove or change the XMP-tiff resolution to match the rounded PPI resolution reported in Photoshop:

exiftool -XMP-tiff:XResolution=300 -XMP-tiff:YResolution=300 -XMP-tiff:ResolutionUnit=inches -ext .png -r 'path to file or top level folder'

Or simply remove the TIFF resolution info:

exiftool -XMP-tiff:XResolution= -XMP-tiff:YResolution= -XMP-tiff:ResolutionUnit= -ext .png -r 'path to file or top level folder'

Either of these commands will make Apple Preview display the resoluiton info as 300 ppi.

(Note: MS Windows would use double quotes around the file path)