I just discovered today that there are two different TIFF tags called “FocalPlaneResolutionUnit.” Tag 41488 goes by this name and is part of the Exif tag set. Accepted values for it are:
- 1 = No absolute unit of measurement
- 2 = Inch
- 3 = Centimeter
Tag 37392 is a TIFF/EP (Electronic Photography) tag (working draft, final version not available online), also used in other raw formats, including DNG. Its accepted values are:
- 1 = Inch
- 2 = Metre
- 3 = Centimetre
- 4 = Millimetre
- 5 = Micrometre
Recently I was sent a TIFF file, as a JHOVE issue, that had a tag 41488 with a value of 4. JHOVE correctly, but perhaps confusingly, reported that the fFocalPlaneResolutionUnit tag had an invalid value.
There are other tags in TIFF/EP that are equivalent, or nearly, to Exif tags. In some cases their values are identically specified, sometimes not. The Exif SubjectLocation tag is numbered 41492 and always has two shorts for its value, giving an X and Y value. The TIFF/EP counterpart is tag 37396, which can also have three shorts (specifying a circle) or four (specifying a rectangle).
I don’t know how this came about, but it’s something to watch out for in software that deals with both Exif and TIFF/EP tags. Some software may accept the EP extensions for Exif tags, but there’s no guarantee this will work.