Canon EOS REBEL T8i Advanced User's Manual page 283

[AUTO] ISO Speed
If the ISO speed is set to [AUTO], the actual ISO speed setting will be displayed in the
viewfinder or on the screen when you press the shutter button halfway.
When [AUTO] is set, the ISO speed is indicated in whole-stop increments. However, the
ISO speed is actually set in finer increments. Therefore, in the image's shooting
information ( ), you may find an ISO speed such as ISO 125 or ISO 640 displayed as
the ISO speed.
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