Disabling creation of arbitrary tags

[BogdanH]

Hi Phil,

Probably has been answered hundreds of times, but even after going thru my "private" Exiftool notices/examples, I ended here. To the question:
Assume there's JPG file with no metadata at all. When I execute:
exiftool -Make=Canon photo.jpg
-exiftool automatically adds some arbitrary tags values. In this case, it's YCbCrPositioning tag I'm talking about. I don't know the exact meaning of this tag (even after reading specs). The thing I've noticed is, that for most of the time, this tag has value "Centered" (but not always: i.e. Canon 7D gives "Co-sited" here). As far I could see, presence of this tag (and it's value) has no influence on viewing/printing the image.
But if I create jpg manually, how to know what's the correct YCbCrPositioning value? I mean, if this value could be important in some cases, then it's better not to define this tag at all, instead of giving it wrong value.
Back to topic...
I know, I can remove this tag after executing above command on empty jpg file. But, how do I know, if tag existed before (thus, has correct value) or it has been added by Exiftool? Is it possible Exiftool would not create this tag automatically? You know, something like:
-exiftool -Make=Canon -Model="LiDE25 scanner" -NoAdd photo.jpg

Thanks for answering,
Bogdan

[Phil Harvey]

Hi Bogdan,

I've never seen any effect of the YCbCrPositioning, but presumably it may have an effect when the image is displayed on some devices.  I can't say I really understand why this is specified in the EXIF and not as part of the JPEG structure.

The EXIF specification states that this tag is mandatory so exiftool assigns an arbitrary value if it isn't specified when the EXIF is created.  As I said, I have never seen this make any difference.

You can force ExifTool to not add the tag with -YCbCrPositioning= when you add new EXIF.  The only trick is that you must know you are adding the EXIF or else this will delete the tag, which maybe isn't ideal, but there it is.

- Phil