Shutter speed is how long the sensor collected light. EXIF stores it in ExposureTime (0x829A) as seconds, an unsigned rational like 1/125, and a second time in ShutterSpeedValue (0x9201) as an APEX number. Both sit in the Exif IFD, which in a TIFF or JPEG is the sub-directory IFD0 points to with tag 0x8769.
Read ExposureTime. It is the value the camera showed you. ShutterSpeedValue exists for the old APEX arithmetic, where exposure is the sum of aperture and time values, and it rarely tells you anything new.
Why 1/125 can turn into 1/128
APEX defines Tv = -log2(t). A true 1/125 s would be 6.97. Camera firmware thinks in powers of two, though, and some (Canon included) simply store 7, which converts back to 1/128 exactly. The "1/125" on the dial is a rounded label for 2^-7 seconds. A Canon EOS RP JPEG shows the mismatch:
$ exiftool -G1 -s -ExposureTime -ShutterSpeedValue photo.jpg
[ExifIFD] ExposureTime : 1/125
[ExifIFD] ShutterSpeedValue : 1/128
$ exiftool -G1 -s -n -ExposureTime -ShutterSpeedValue photo.jpg
[ExifIFD] ExposureTime : 0.008
[ExifIFD] ShutterSpeedValue : 0.0078125
$ exiftool -v1 photo.jpg | grep -E "ExposureTime|ShutterSpeedValue"
| | 0) ExposureTime = 0.008 (1/125)
| | 13) ShutterSpeedValue = 7 (458752/65536)Nothing is wrong with the file. Even -n gives you seconds for ShutterSpeedValue, because ExifTool's value conversion undoes the APEX step. Only verbose output shows the stored 7.
Long exposures go negative
ShutterSpeedValue is a signed rational, and anything longer than one second has a negative APEX value. Written with ExifTool, a 2 s exposure is stored as -1 and a 30 s exposure as about -4.91. ExposureTime has no sign to worry about: 30 s is just 30/1.
If you need to know the shutter was in Bulb mode, check ExposureProgram, where some Canon models write the non-standard value 9. Canon also keeps a BulbDuration tag in its maker notes.
Changing the shutter speed fields
The EXIF editor accepts both fields in JPEG and TIFF files as text such as 1/800, and ExifTool turns that into the right rational or APEX value. On the desktop, Windows Explorer and macOS Preview both show the exposure time; see viewing metadata on Windows and Mac. The aperture entry covers the matching APEX math for f-numbers.