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Please note, I'm not asking what resolution is!

I've processed a raw image and exported from Photoshop, and I've just noticed something in the image details that I've never seen before (I don't think):

image showing properties of the file

Second from last value shown is "Resolution unit" and gives a value of "2". What does this represent?

I found a post on another site which indicates the number means the units of measurement — inches or cm or absolute — is this correct? Can't seem to find much information on this.

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This is in the Exif standard for metadata, on page 26:

ResolutionUnit The unit for measuring XResolution and YResolution. The same unit is used for both XResolution and YResolution. If the image resolution in unknown, 2 (inches) is designated. 

Tag            =             296 (128.H)                  
Type           =             SHORT                
Count          =             1              
Default        =             2             
   2           =             inches               
   3           =             centimeters               
   Other       =             reserved

So, indeed, "2" is inches and "3" is cm — but "1" and other values are "reserved", which is to say not supposed to be used.

The Windows Explorer dialog is interpreting other values (e.g., "ColorSpace = 1" has become "Colour representation sRGB"), it's a bit odd that this value is shown as a number rather than just "inches" — or, indeed, left out, because the Horizontal Resolution and Vertical Resolution (from XResolution and YResolution) are labeled "dpi" — where the "i" is inches.

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  • \$\begingroup\$ So, wait. If the resolution is in inches, does it mean that according to the image properties shown, the image is 6720 pixelinches * 4480 pixelinches, or that it has a resolution of 300 Dot Per Inches inches? \$\endgroup\$
    – motoDrizzt
    Commented Feb 27, 2018 at 22:38
  • \$\begingroup\$ Neither of those. The resolution in the file is the horizontal and vertical resolution — 300 in this case — and the units are specified here. The software is apparently just assuming dpi when it could be d/cm (without checking this value). \$\endgroup\$
    – mattdm
    Commented Feb 27, 2018 at 22:59
  • \$\begingroup\$ Of course, all of this is highly academic since that value is basically meaningless. \$\endgroup\$
    – mattdm
    Commented Feb 27, 2018 at 23:00
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From MSDN Knowledge Database

System.Image.ResolutionUnit Indicates the resolution units. Used for images with a non-square aspect ratio, but without meaningful absolute dimensions. 1 = No absolute unit of measurement. 2 = Inches. 3 = Centimeters. The default value is 2 (Inches).

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DPI means 'dots per inch' and is mostly irrelevant unless you're going to print the image.

At that point, if you have 300 dpi, then you're going to get an image on paper that translates your file resolution to determine the desired image size in inches AT 300 dpi on the print.

It's inversely proportional to the finished print. The higher the DPI, the smaller the printed image, and vice versa. (For a given image resolution.)

It's just a conversion ratio, but in practice it also matters little to most people (like me!) when printing. I manually set the target paper image dimensions I'm after, and let the software do the conversion.

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