Why there are 2 focal lengths in EXIF output

[SK]

Hi, I used the exifTool to spit out all the meta data from my .CR2 file. I see 2 focal lengths:

FocalLength => 100.0 mm
FocalLength (1) => 100 mm

What is the difference?

[Phil Harvey]

Things like focal length are stored in many places in Canon files.  I don't know the difference, but you can add the -G1 option to see where they are coming from.

- Phil

[Hayo Baan]

Likely one is in the makernotes while the other is in the exif data.

[SK]

Hi All, thank you.

Phil, as per your suggestion used the -G flag and I see the following groups of data:

1. [File]
2. [EXIF]
3. [MakerNotes]
4. [Composite]

I am familiar with EXIF and MakerNotes (camera specific) but what are groups [FILE] & [COMPOSITE]?.

Again my assumption is the the EXIF tags will not change in the future (I understand more might be added to the standard) -- is my assumption correct? Can I create table based on existing EXIF tags and can be assured these tags will always be the SAME? or best is to use XML - free format. This is in case I scan all my photo's and create a photo data.

If I want to create a copyright & keyword tags... where should it be logically placed?

Thanks much. Learning a lot!

[Phil Harvey]

I am familiar with EXIF and MakerNotes (camera specific) but what are groups [FILE] & [COMPOSITE]?.

The File family 1 group is both system information and basic information found in the specific file type.  Use -G1 to distinguish between these.

Composite tags are derived from the values of other tags.

Again my assumption is the the EXIF tags will not change in the future (I understand more might be added to the standard) -- is my assumption correct?

This is true for standard EXIF tags, but ExifTool also deals with non-standard ones.

Can I create table based on existing EXIF tags and can be assured these tags will always be the SAME?

No.  ExifTool doesn't differentiate between standard and non-standard EXIF tags.

If I want to create a copyright & keyword tags... where should it be logically placed?

I would recommend XMP for this.

- Phil

[SK]

Hi Phil, thank you. Can i WRITE XMP data on a raw file using ExifTool? I am familiar with Perl. Any examples or URL where I can look at code snippet? My assumption is XMP is considered metadata.

[Phil Harvey]

Yes you can write XMP to CR2 files.  You don't need to know Perl.  The command-line syntax is like this:

exiftool -subject="keyword1" -subject="keyword2" -rights="Copyright 2016 Your Name" FILEorDIR

There are lots of other examples in this section of the application documentation.

But if you want to do this from your own Perl script using the ExifTool API, look here for documentation and examples.

- Phil

[SK]

How do i write a tag based on XMP namespace for example, I want specifically to write XMP tag description....is it something like this?

exiftool -xmp:Description="something" filename

[StarGeek]

That would be the correct way to do it.