Shutter Speed
0x829aAlso: ExposureTime
How long the sensor was exposed to light.
Shutter speed is how long the shutter stayed open, in seconds. Fast speeds like 1/1000 s freeze motion; slow speeds like 1/15 s or several seconds blur it and gather more light.
EXIF stores it as a rational (e.g. 1/250). Readers usually show fractions for anything under a second.
ExposureTime (0x829a) is the honest one, in seconds. ShutterSpeedValue (0x9201) is again an APEX logarithm and rounds badly past a second. A phone in night mode reports the time of a single sub-frame, so a file that says 1/17 s may well be a shot you stood still for three seconds to take.
Want to see this tag in your own photo? Open the EXIF Viewer and drop an image — every tag is decoded in your browser. To take this tag back out again, remove the EXIF data losslessly.