Sony a9 Instruction Manual page 120

SLOW (Slow)/SLOWER (Slower): The ISO sensitivity will start to change at
shutter speeds slower than [Standard], so you can shoot images with
less noise.
1/16000 – 30": The ISO sensitivity starts to change at the shutter speed
you have set.
Hint
• The difference in shutter speed at which ISO sensitivity starts to change
between [Faster], [Fast], [Standard], [Slow], and [Slower] is 1 EV.
Note
• If the exposure is insufficient even when the ISO sensitivity is set to
[ISO AUTO Maximum] in [ISO AUTO], in order to shoot with an appropriate
exposure, the shutter speed will be slower than the speed set in
[ISO AUTO Min. SS].
• In the following situations, the shutter speed may not function as set:
– When the maximum shutter speed has changed based on the
[ Shutter Type] setting.
– When using the flash to shoot bright scenes. (The maximum shutter speed
is limited to the flash-syncing speed of 1/250 seconds.)
– When using the flash to shoot dark scenes with the [Flash Mode] set
to [Fill-flash]. (The minimum shutter speed is limited to the speed
automatically determined by the camera.)
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