Solution
Select [0: Disable] to change the [C.Fn II -3: Highlight tone priority] settingSelect [1: On] to change the [C.Fn II -2: ISO expansion] setting
If you set [C.Fn II -3: Highlight tone priority] to [1: Enable] on your camera, the settable ISO range will be limited to ISO 200-12800 (up to ISO 6400 for movies), so "ISO 100" or "H"(Still photo: equivalent to ISO 25600, Movie: equivalent to ISO 12800) cannot be selected.
However, if you set [C.Fn I -2: ISO expansion] to [1: Enable], "H" (Still photo: equivalent to ISO 25600, Movie: equivalent to ISO 12800) will become selectable.
If you wish to make the "ISO 100" or "H"(Still: equivalent to ISO 25600, Movie: equivalent to ISO 12800) setting (ISO expansion), change the setting as follows.
Select [0: Disable] to change the [C.Fn II -3: Highlight tone priority] setting
The [D+] icon will be displayed on the LCD panel when highlight tone priority is enabled.
1. Set the power to ⋖ON⋗.
2. Press the ⋖MENU⋗ button to display the menu.
3. Press the ⋖Main Dial⋗ to select [Custom Functions(C.Fn)] under the [ ] tab, then press the ⋖
/SET⋗ button.
4. Press the ⋖Main Dial⋗ to select Custom Function [3: Highlight tone priority], then press the ⋖/SET⋗ button.
5. Select [0: Disable], and then press the ⋖/SET⋗ button.
6. Press the shutter button halfway, and the camera will be ready for shooting.
Confirm that [D+], which indicates Highlight tone priority, is not displayed on the LCD monitor.
Caution
About Highlight tone priority
Select [1: On] to change the [C.Fn II -2: ISO expansion] setting
1. Press the ⋖MENU⋗ button to display the menu.
2. Press the ⋖Main Dial⋗ to select [Custom Functions(C.Fn)] under the [ ] tab, then press the ⋖
/SET⋗ button.
3. Press the ⋖Main Dial⋗ to select Custom Function [2: ISO expansion], then press the ⋖/SET⋗ button.
4. Select [1: On], and then press the ⋖/SET⋗ button.
5. Press the shutter button halfway, and the camera will be ready for shooting.
When you set the ISO speed, you will be able to set "H (equivalent to ISO 25600)" for still photos and "H (equivalent to ISO 12800)" for movies.
Caution
Note that if [C.Fn-3: Highlight tone priority] is set to [1: Enable], "H" cannot be set.