Unexpected changes to X Resolution and Y Resolution

[Archive]

[Originally posted by actcms on 2007-03-09 18:02:34-08]

I'm new to ExifTool. Running under windows using a freshly scanned image from an epson 3490 (scanned from a 35mm negative at 3200x3200 resolution with supplied epson software and saved directly to disk).

Output from -w txt option on freshly scanned file:

ExifTool Version Number         : 6.78
File Name                       : N0038-025.jpg
Directory                       : C:\Exif Tests\1978-08 - N0038
File Size                       : 1246 kB
File Modification Date/Time     : 2007:02:25 23:59:32
File Type                       : JPEG
MIME Type                       : image/jpeg
JFIF Version                    : 1.1
Resolution Unit                 : inches
X Resolution                    : 3200
Y Resolution                    : 3200
Image Width                     : 2826
Image Height                    : 4480
Image Size                      : 2826x4480

I then ran -CreateDate='1978:08:01'

Output from -w txt option on date stamped file:

ExifTool Version Number         : 6.78
File Name                       : N0038-025.jpg
Directory                       : C:\Exif Tests\1978-08 - N0038
File Size                       : 1247 kB
File Modification Date/Time     : 2007:03:09 17:33:22
File Type                       : JPEG
MIME Type                       : image/jpeg
JFIF Version                    : 1.1
X Resolution                    : 72
Y Resolution                    : 72
Resolution Unit                 : inches
Y Cb Cr Positioning             : Centered
Exif Version                    : 0220
Create Date                     : 1978:08:01 00:00:00
Components Configuration        : YCbCr
Flashpix Version                : 0100
Color Space                     : Uncalibrated
Image Width                     : 2826
Image Height                    : 4480
Image Size                      : 2826x4480

I am interested to know why the resolution has changed to 72x72.

I have visually compared the changed file with the saved original in Photoshop 7 and can see no difference.

[Archive]

[Originally posted by exiftool on 2007-03-09 18:24:32-08]

This is related to
https://exiftool.org/faq.html#Q3" target="_blank">FAQ #3.
If you use the -a and -g1 options I could tell for sure, but
my guess is that the Epson software only writes the JFIF resolution information.
When you ask ExifTool to add EXIF information, it must fill in the mandatory
X/YResolution information if not specified.  By default, it assumes 72 DPI.

If I am correct, the JFIF resolution information will be unchanged, and
you are just seeing the EXIF resolution which takes precedence when extracting
information with ExifTool (unless you use the -a option to see them both).

I hope this makes sense.

When using photoshop, there is another resolution value stored in the photoshop
information.  I would guess that Photoshop will use this value preferentially, or
from what you describe will take the JFIF resolution over the EXIF resolution otherwise.

Yes, this can be confusing. https://exiftool.org/forum/Smileys/default/wink.gif" alt="Wink" border="0" />

- Phil

[Archive]

[Originally posted by actcms on 2007-03-09 18:52:07-08]

Spot on - the original resolution info is stored in JFIF and is still there after the date stamp. I just started with ExifTool yesterday and I need to do more work on the available options - I'm sure I will be able to use ExifTool to copy the JFIF info into the EXIF tags once I do a bit more study. Thanks for your prompt response - I'm now up and running. "95% of the fun is in the learning..."

[Archive]

[Originally posted by exiftool on 2007-03-09 20:34:35-08]

Great.

This confusion has occurred before, but at the time I couldn't think of
an easy way to use the JFIF resolutions as the default mandatory EXIF
resolutions when adding new EXIF information.  However, I have done
a bit more thinking about this and I have figured out a way to do it.

So look for ExifTool 6.80 to use the JFIF resolutions if
available when creating a new EXIF segment in JPEG images.

This should future problems like this.

The https://exiftool.org/Image-ExifTool-6.80.tar.gz" target="_blank">
6.80 pre-release has been updated with this new feature.

- Phil