Unable to Set the ISO Speed to "100"
Solution
If the "C.Fn II -3 Highlight tone priority" custom function is set for your camera, the settable ISO speed (image sensor sensitivity) range will be 200 - 3200.
To set the ISO speed to "100", set this custom function to [0: Disable].
Please see the Instruction Manual that is supplied with your camera for details about Custom Function settings.
IMPORTANT
C.Fn II -3 Highlight tone priority
When "C.Fn II -3 Highlight tone priority" is set to [1: Enable], the highlight detail will be improved. The dynamic range is expanded from the standard 18% gray to bright highlights. The gradation between the grays and highlights becomes smoother.
With setting [1: Enable], noise in the shadow areas may be slightly more than usual.