photoshop automatic measurement scale

[CVHManchester]

I am scanning prints at a number of resolutions, 1600 to 3000 dpi saved as TIFF. This metadata is recorded in the image Exif file.

When I open the file in Photoshop, the recorded resolution is indicated in the bottom left corrner with the image size in mm. I currently have 3 files open, all with different scan resolutions, correctly indicated.

However, the mm size and any measurement I do with the ruler appears to converted from pixel dimensions to mm using 1200 dpi, which appears to be the Default conversion factor.

How can I ensure that Photoshop uses the actual resolution of the image file when it is loaded?

Thank you

[c.pfaffenbichler]

Could you please post meaningful screenshots to illustrate? 

Edit: Seems to work here. The screenshot shows two 100mm x 100mm images with different Resolutions in which I manually marked the diagonal. 

[CVHManchester]

Thank you for the prompt reply.

Above is a typical (if small) print scanned at 1600 ppi.

Below is a print scanned at 1800 ppi.

I have drawn a ruler line on the 80 mm scale, which is a fairly accurate microscope scale. The reported widths are different from each other and both wrong.  It is quite possible that I have set a wrong setting somewhere, but I don't know what it could be.

What measurement setting is your system? It seems to work.

[Semaphoric]

If you are including a scale in every image, you could use the Ruler's Use Measurement Scale feature (in the Options). With the box checked, go to Image > Analysis > Set Measurement Scale > Custom. You enter the pixel length, physical length, and name of units.

Using your first screenshot, I got this:

With this, the Ruler will read out the physical size. For instance, I measure the word Northampton as just over 22mm long.

You can save custom Measurement Scales as presets, but I don't know if they are embedded in the document file.

[CVHManchester]

Thank you.

The trouble is that I only recently started to include a scale in the scan image.  Then I found out that the physical size measurement readout did not match the physical size. So I have a couple of thousand scans without a scale included.

Which I why I want (expect) Photoshop to do the automatic scaling.

Presumably when you created then saved then opened the squares you show above, Photoshop must have read the ppi in from the file metadata.

Any idea how this can be achieved with my files?

[c.pfaffenbichler]

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Which I why I want (expect) Photoshop to do the automatic scaling.

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What do you mean exactly? 

Do you expect Photoshop do »recognize« the ruler to recalculate the resolution (without resampling)? 

And I am not sure I understand your process: 

Did you scan at 100% and just change the resolution or did you scale in scanning and change the resolution? 

Could you provide a file? 

[CVHManchester]

Thank you. I shall try to be more explicit.

I scan these paper impressions on an Epson 10000XL, producing TIFF files 1-3 GB.  Because of the memory limitations of the scanner and TIFF, ca. 3 GB, the scanner resolution needs to be adjusted 1200-3000 dpi depending on the size of the impression. The scanner's (i.e. image's) resolution is recorded in the Exif metadata.

When I load the image file into Photoshop, I want Photoshop to recognise the resolution of the image and use that resolution in calibrating the ruler.  Thus a measurement on the ruler should match the measurement on the original impression.  As you can see above, the ruler measurements on the loaded image files are well out.

Any manipulation of the image in Photoshop is confined to straightening the image or cropping to the margins.  The resolution is not changed.

I want to use the ruler to read accurate measurements of features on the image, as part of the documentation of the impression.

[c.pfaffenbichler · Accepted solution]

And you did definitely not set scaling other than 100% in the Scanner software? 

Can you provide a scan? 

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When I load the image file into Photoshop, I want Photoshop to recognise the resolution of the image

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Photoshop does that, so I supect some other problem. 

[c.pfaffenbichler]

I tried reversing the screenshot with »Northampton« and when I draw a rectangle to get the pixel width of the 8cm-ruler I get about 4150px. 

At 1600ppi that is simply not 8cm. 

What number of pixels do you get for that width? 

[CVHManchester]

Mea culpa

You asked the pertinent question.  So I investigated.

The scanning software SilverFast records in the metadata a desired "Preset" resolution, not the actual scanned resolution.  This incorrect metadata then skewed Photoshop's response.

Unfortunately, I haven't found a way to discover if the recorded resolution is the actual scanned resolution of an image, in order to redefine it in the metadata somehow.

As a novice, I shall have to be more careful in understanding all the implications of an "expert" setting.

Thank you for your assistance in finding my error and correcting my practice.

[c.pfaffenbichler]

Can you provide a file? (Just the scale should be enough.) 

Maybe one can access the metadate setting inside Photoshop.