exiftool reports 0.011mm, but some Web resources say 0.019mm. Which one is more correct? Any source for the value?
ExifTool calculates this as D/1440, where D is the focal plane diagonal in mm. The constant 1440 is somewhat subjective, and different sources use different values. You can set this constant to whatever you want by overriding this tag in the config file (https://exiftool.org/config.html).
- Phil
I have just realized that it can change with the image size, so it is also calculated...