Is it easy to fix or remove "FocalLengthIn35mmFormat=0mm"?

[quickshot]

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., 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?

[StarGeek]

Composite tags do not exist in the file. They are created on the fly based upon other tags. See the Composite tags page 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 to suppress the creation of backup files.  Add -r to recurse into subdirectories.

[quickshot]

This sounds great, but it did not work when I tried it:

Code Select Expand

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:

Code Select Expand

C:\>exiftool -if "$FocalLengthIn35mmFormat eq '0 mm'" -FocalLengthIn35mmFormat= "IMG20220812181313.jpg"
    1 image files updated

C:\>exiftool -FocalLengthIn35mmFormat IMG20220812181313.jpg
exiftool -FocalLengthIn35mmFormat IMG20220812181313.jpg

C:\>

[StarGeek]

Oops, my mistake. Yes, the dollar sign was missing from my command.