Thank you for this. Now I understand why I could not find any references about generating exact NDVI values using images taken using modded cameras. You are right, the GoPro image exif data is very basic and I don’t think calibration is possible with it.
So if I understand your explanation correctly, based on the image I have, I am only limited to a graphical output of the NDVI. Meaning that I could put out a colorized LUT, just to show that this particular area might be more dense/healthy than others, but I could not derive exact NDVI values for comparison. Is this right?