This article may apply to other Canon cameras, but has only been tested on the Canon R100.
Change your Canon R100 video mode to "manual" (or back to automatic)
Follow these steps:
Disconnect the USB cord from the camera, if it is plugged in
Change the dial on the top of the camera to the video icon
Press the Q/set button in the middle of the D-pad on the back of the camera - the video icon should now be selected on the screen; if not, move around to select it
Press the Q/set button again to select it
In the popup screen, change to Move Manual Exp. (or auto) as desired
Change the dial on the top of the camera back to 'M' mode
Then reconnect your camera's USB cable.
When in movie manual exposure mode, you cannot change the video Exposure value - it is locked as in the screenshot below

