VideoKeys "-und-FI" and non-"-und-FI"

[wywh]

I am puzzled about the purpose of those seemingly duplicate "-und-FI" VideoKeys and whether I need to preserve either or both of them? Or not mess with them at all.

For example an iPhone movie:

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exiftool -a -G1 -s -VideoKeys:All 2024-1221-1741-55.mov
[VideoKeys]     LensModel-und-FI                : iPhone 16 Pro back camera 6.765mm f/1.78
[VideoKeys]     FocalLengthIn35mmFormat-und-FI  : 25
[VideoKeys]     LensModel                       : iPhone 16 Pro back camera 6.765mm f/1.78
[VideoKeys]     FocalLengthIn35mmFormat         : 25
Only "-und-FI" (or maybe in fact the non-"-und-FI"?) seems to really be present in the original iPhone movie? Adding -v3 seems to indicate the other sibling is not real?

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  | | | ItemList (SubDirectory) -->
  | | | + [ItemList directory]
  | | | | LensModel-und-FI = iPhone 16 Pro back camera 6.765mm f/1.78
  | | | | FocalLengthIn35mmFormat-und-FI = 25
-v3:
(synthesized default-language tag for QuickTime:LensModel)
(synthesized default-language tag for QuickTime:FocalLengthIn35mmFormat)
"-und-FI" tags refuse to delete from the original iPhone movie:

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exiftool -overwrite_original -VideoKeys:LensModel-und-FI= -VideoKeys:FocalLengthIn35mmFormat-und-FI= 2024-1221-1741-55.mov
    0 image files updated
    1 image files unchanged

exiftool -a -G1 -s -VideoKeys:All 2024-1221-1741-55.mov
[VideoKeys]     LensModel-und-FI                : iPhone 16 Pro back camera 6.765mm f/1.78
[VideoKeys]     FocalLengthIn35mmFormat-und-FI  : 25
[VideoKeys]     LensModel                       : iPhone 16 Pro back camera 6.765mm f/1.78
[VideoKeys]     FocalLengthIn35mmFormat         : 25
But deleting their non-"-und-FI" siblings surprisingly deletes all those tags:

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exiftool -overwrite_original -VideoKeys:LensModel= -VideoKeys:FocalLengthIn35mmFormat= 2024-1221-1741-55.mov
    1 image files updated

exiftool -a -G1 -s -VideoKeys:All 2024-1221-1741-55.mov
Warning: [minor] The ExtractEmbedded option may find more tags in the media data [x24] - 2024-1221-1741-55.mov

exiftool -a -G1 -s -ee -VideoKeys:All 2024-1221-1741-55.mov
[empty]
On the other hand, if I copy those tags to another movie, then both siblings seem to be real with no "synthesized" in v3 output:

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exiftool -m -overwrite_original -api QuickTimeUTC=1 -tagsFromFile 2024-1221-1741-55.mov -All:All movie.mp4

exiftool -a -G1 -s -ee -VideoKeys:All movie.mp4
[VideoKeys]     FocalLengthIn35mmFormat         : 25
[VideoKeys]     FocalLengthIn35mmFormat-und-FI  : 25
[VideoKeys]     LensModel                       : iPhone 16 Pro back camera 6.765mm f/1.78
[VideoKeys]     LensModel-und-FI                : iPhone 16 Pro back camera 6.765mm f/1.78

  | | | ItemList (SubDirectory) -->
  | | | + [ItemList directory]
  | | | | FocalLengthIn35mmFormat = 25
  | | | | FocalLengthIn35mmFormat-und-FI = 25
  | | | | LensModel = iPhone 16 Pro back camera 6.765mm f/1.78
  | | | | LensModel-und-FI = iPhone 16 Pro back camera 6.765mm f/1.78
And then the "-und-FI" can be separately deleted. And Photos.app is still happy to display the Lens info:

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exiftool -overwrite_original -VideoKeys:LensModel-und-FI= -VideoKeys:FocalLengthIn35mmFormat-und-FI= movie.mp4
    1 image files updated

exiftool -a -G1 -s -VideoKeys:All movie.mp4
[VideoKeys]     FocalLengthIn35mmFormat         : 25
[VideoKeys]     LensModel                       : iPhone 16 Pro back camera 6.765mm f/1.78
But then deleting the non-"-und-FI" surprisingly deletes both:

Code Select Expand

exiftool -a -G1 -s -VideoKeys:All movie.mp4
[VideoKeys]     FocalLengthIn35mmFormat         : 25
[VideoKeys]     FocalLengthIn35mmFormat-und-FI  : 25
[VideoKeys]     LensModel                       : iPhone 16 Pro back camera 6.765mm f/1.78
[VideoKeys]     LensModel-und-FI                : iPhone 16 Pro back camera 6.765mm f/1.78

exiftool -overwrite_original -VideoKeys:LensModel= -VideoKeys:FocalLengthIn35mmFormat= movie.mp4
    1 image files updated
exiftool -a -G1 -s -VideoKeys:All movie.mp4
[empty]
...still with me? So it seems one or the other of those tags is a master and deleting it deletes also the other, right? And it is better not to mess with them?

- Matti