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I have a resized JPG of a photo that still contains the original EXIF data. How can I calculate the original resolution (in pixels) the camera was set to when this photo was taken? In the EXIF, I see the following possibly relevant info:

EXIF:
Model                       NIKON D850
XResolution                 240
YResolution                 240
ResolutionUnit              inches
FocalPlaneXResolution       2301.324615
FocalPlaneYResolution       2301.324615
FocalPlaneResolutionUnit    cm
FocalLength                 78.0 mm
FocalLengthIn35mmFormat     78 mm

XMP:
CropTop 0
CropLeft    0
CropBottom  0.932059
CropRight   0.932058

Can from this data the resolution be derived the picture was originally taken at?

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    \$\begingroup\$ Not from those tags, but if the full EXIF data is really all still there, there might be other pertinent data (especially if there's a MakerNote section). Either keep looking, or post more of the data. \$\endgroup\$
    – twalberg
    Commented Dec 5, 2019 at 1:32
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    \$\begingroup\$ What's the purpose of this exercise? \$\endgroup\$
    – xiota
    Commented Dec 5, 2019 at 2:30
  • \$\begingroup\$ Check the Nikon D850 specs (click open the Image Sensor section) for the possible original sizes. \$\endgroup\$
    – StarGeek
    Commented Dec 5, 2019 at 4:30
  • \$\begingroup\$ @twalberg given that both FocalLength and FocalLengthIn35mmFormat are equivalent, doesn't that mean that full sensor area (36mm * 24mm) was used? and documentation for FocalPlaneXResolution states this is the resolution in the focal plane AFTER processing (so, the actual file resolution). Wouldn't that mean X Resolution was 2301.324615 / 1cm * 36mm and Y Resolution was 2301.324615 / 1cm * 24mm? \$\endgroup\$
    – matthias_buehlmann
    Commented Dec 5, 2019 at 13:39
  • \$\begingroup\$ @user1282931 Thats a good first approximation, but there should be other tags that have the actual pixel dimensions - "Image Height" and "Image Width" for example (spelling may vary depending on what tool you are using - between exiftool and exiv2, for instance, one produces "Image Height" while the other displays "ImageHeight" without the space). I'm a Canon user, though, and the tags that are actually populated may differ from what Nikon does... \$\endgroup\$
    – twalberg
    Commented Dec 5, 2019 at 14:22

2 Answers 2

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The XResolution and YResolution tags are supposed to contain the image resolution. Unfortunately they're meaningless for a camera image, because the camera doesn't have a fixed resolution - it varies based on the distance to the subject. They will be filled in with a constant, typically 72 DPI. Those fields were put in the EXIF specification for things like scanners that have a useful resolution to fill in.

https://www.w3.org/2003/12/exif/

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The size of the original image in pixels appears to be kept in two tags:

  • PixelXDimension (A002)
  • PixelYDimension (A003)
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