[Bug 764093] tag: exif: wrong parsing of ISO_SPEED and PHOTOGRAPHIC_SENSITIVITY tags

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=764093

Thiago Sousa Santos <thiagossantos at gmail.com> changed:

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--- Comment #8 from Thiago Sousa Santos <thiagossantos at gmail.com> ---
Not sure I agree with this patch.

0x8827 is EXIF_TAG_PHOTOGRAPHIC_SENSITIVITY
0x8833 is EXIF_TAG_ISO_SPEED
And also
0x8830     SensitivityType

And in this particular file we have
0x8827 ISO                             : 100
0x8830 Sensitivity Type                : Recommended Exposure Index
0x8832 Recommended Exposure Index      : 100
(No 0x8833 present)

So, the reason why ISO is not extracted here is because we limited getting
0x8827 (photographic sensitivity) only when 0x8830 tells us that it is ISO
Speed (value 3).

0x8827 only has a meaning after we parse 0x8830. That is why it is dependent on
the former.

Now, the real problem here is that we only accept 0x8830 when it is an ISO
Speed. It can also be a SOS (Standard Output Sensitivity), REI (Recommended
Exposure Index), ISO Speed or any combination of all.

Quick reading about those different values seem to indicate they are all very
similar, it seems REI might just be a non-official per-company setting that is
similar to ISO on their sensors.

Anyway, I think the question here is how to map other interpretations of
SensitivityType to ISO, if possible.

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