Unable to Set the ISO Speed to "100" (EOS 5D Mark III)
24-Feb-2012
8201359500
Solution
As indicated below, if [ 
:Highlight tone priority] under the [ 
] tab is set to [Enable], the settable ISO speed (image sensor sensitivity) range will be ISO 200-25600 (or up to 12800 for movie shooting). You cannot select "L" (ISO 50), ISO 100/125/160, "H1" (ISO 51200), and "H2" (ISO 102400).
:Highlight tone priority] under the [ ] tab is set to [Enable], the settable ISO speed (image sensor sensitivity) range will be ISO 200-25600 (or up to 12800 for movie shooting). You cannot select "L" (ISO 50), ISO 100/125/160, "H1" (ISO 51200), and "H2" (ISO 102400).
To set the ISO speed to "100", set this custom function to [0: Disable].
IMPORTANT
About Highlight tone priority
When "Highlight tone priority" is set to [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 [Enable], noise may be slightly more than usual.