The app of my mobile phone ("MediaTek Camera Application") creates the nonsense EXIF tag
[ExifIFD] FocalLengthIn35mmFormat : 0 mm
But there also is a
[Composite] FocalLength35efl : 4.7 mm
I'm not sure the 4.7mm is correct for 35mm, because it would be an ultimate fisheye, I guess.
(My older phone added "[Composite] FocalLength35efl : 3.8 mm (35 mm equivalent: 26.0 mm)")
So my question is: Can I remove such bad tags (i.e.: not all "FocalLengthIn35mmFormat", but only those that have a value of zero), or can I easily fix the tags (e.g. assuming for example that 4.7mm is the native focal length (and the factor is about 6.8), then the 35mm local length would be 32mm, a moderate wide-angle lens).
That is, can exiftool read the value of a tag, do some maths with it and save the result to a possibly different tag?
Composite tags do not exist in the file. They are created on the fly based upon other tags. See the Composite tags page (https://exiftool.org/TagNames/Composite.html) to find the underlying tags that create them.
In the case of FocalLength35efl, there is a chain of tags the creates it. It is computed based upon the FocalLength and ScaleFactor35efl. But ScaleFactor35efl is itself another Composite tag, which is based upon a bunch of other tags. One of those tags is the FocalLengthIn35mmFormat tag. And since that has a value of 0, there is a cascade up the chain of problematic values.
You can remove problem FocalLengthIn35mmFormat tags with a command like
exiftool -if "FocalLengthIn35mmFormat eq '0 mm'" -FocalLengthIn35mmFormat= /path/to/files/
This command is for Windows CMD. If you are on Mac/Linux, swap the double/single quotes i.e double quotes are inside the single quotes.
This command creates backup files. Add -Overwrite_Original (https://exiftool.org/exiftool_pod.html#overwrite_original) to suppress the creation of backup files. Add -r (https://exiftool.org/exiftool_pod.html#r-.--recurse) to recurse into subdirectories.
This sounds great, but it did not work when I tried it:
C:\>exiftool -if "FocalLengthIn35mmFormat eq '0 mm'" -FocalLengthIn35mmFormat= "IMG20220812181313.jpg"
1 files failed condition
C:\>exiftool -FocalLengthIn35mmFormat IMG20220812181313.jpg
exiftool -FocalLengthIn35mmFormat IMG20220812181313.jpg
Focal Length In 35mm Format : 0 mm
Reading the manual it seems your solution lacks a dollar sign for the variable; after I added one, it worked:
C:\>exiftool -if "$FocalLengthIn35mmFormat eq '0 mm'" -FocalLengthIn35mmFormat= "IMG20220812181313.jpg"
1 image files updated
C:\>exiftool -FocalLengthIn35mmFormat IMG20220812181313.jpg
exiftool -FocalLengthIn35mmFormat IMG20220812181313.jpg
C:\>
Oops, my mistake. Yes, the dollar sign was missing from my command.