ISO in EXIF is the sensitivity setting used for the shot, stored in tag 0x8827 of the Exif IFD. The tag has had three names: ISOSpeedRatings in EXIF 2.2, PhotographicSensitivity since EXIF 2.3, and simply ISO in ExifTool, which is the name you type on the command line and the one ExifTool prints with -G as EXIF:ISO.
A 16-bit field in a six-digit ISO world
The type is SHORT, an unsigned 16-bit integer, so the largest value it can hold is 65535. Current bodies go well past that, and ExifTool refuses to write anything bigger:
$ exiftool -overwrite_original -ISO=102400 photo.jpg
Warning: Value above int16u maximum for IFD0:ISO
0 image files updated
1 image files unchangedEXIF 2.3 fixed this by adding 32-bit companions instead of widening the old tag: StandardOutputSensitivity (0x8831), RecommendedExposureIndex (0x8832) and ISOSpeed (0x8833). SensitivityType (0x8830) says which of those definitions the ISO value follows:
| SensitivityType | Meaning |
|---|---|
| 0 | Unknown |
| 1 | Standard Output Sensitivity |
| 2 | Recommended Exposure Index |
| 3 | ISO Speed |
| 4 to 7 | Combinations of the above |
A camera shooting above 65535 has to fall back on those 32-bit tags for the real number, so for very high ISO shots, read all of them alongside ISO.
What a normal file looks like
A Canon EOS RP JPEG at ISO 400:
$ exiftool -G1 -s -ISO -SensitivityType -RecommendedExposureIndex photo.jpg
[ExifIFD] ISO : 400
[ExifIFD] SensitivityType : Recommended Exposure Index
[ExifIFD] RecommendedExposureIndex : 400
$ exiftool -G1 -s -n -ISO -SensitivityType -RecommendedExposureIndex photo.jpg
[ExifIFD] ISO : 400
[ExifIFD] SensitivityType : 2
[ExifIFD] RecommendedExposureIndex : 400The ISO tag may also hold several numbers. That is rare, but it breaks parsers that expect a single integer.
Auto ISO is not recorded in standard EXIF. You only get the value the camera picked; whether it was chosen automatically lives in the maker notes, if anywhere.
Reading and editing ISO
iPhone photos keep ISO in the same EXIF tag even inside a HEIC file, and the Photos app shows it in the info panel (viewing metadata on iPhone). For JPEG and TIFF, the EXIF editor writes EXIF:ISO as a plain number. SensitivityType and the 32-bit tags are not in our list, so set them with ExifTool, for example exiftool -SensitivityType#=2 -RecommendedExposureIndex=102400 photo.jpg. ISO sits next to shutter speed and aperture in every exposure calculation.