Nikon D810 EXIF

[midori]

I am using latest versions of exiftool.exe with elatest exiftoolGUI.exe and Nikon firmware.

Regarding Nikon field 'secondaryslotfunction'  I observe changes to exif data for no apparent reason.
As I scroll thru images from same session, I read either of the 4 data below:-
a) NEF Primary + JPG Secondary   (this is how the D810 is set up by the way)
b) Backup
c) Unknown (0x1)
d) Overflow

Does anyone know if this is a Nikon issue or exiftool issue?  The only external factors I can see is that camera is rotated from Horizontal (normal)  to Rotate 90CW, and finally back to Horizontal (normal)

[Phil Harvey]

It is likely that the location of this metadata changed with a firmware update.  What is the FirmwareVersion for your camera?  Try using the -U option and taking some pictures with just the SecondarySlotFunction changed, and see if you can figure out what unknown tag corresponds to this information. 

- Phil