? did^n't understand "bracketing" term. I am newbie for professional.. :'(
Bracketing is taking more than one shot, changing exposure, just like you are trying to do.
Try
http://chdk.wikia.com/wiki/CHDK_User_Manual#Bracketing_in_continuous_modeSimple instructions:
in CHDK "extra photo operations" select "Bracketing in continuous mode"
set tv bracket value to 2
set bracketing type to +/-
leave the menu
leave CHDK alt mode
in the canon FUNC menu, select continuous mode.
When you want to take a picture, hold down the shutter for three shots. First will be your normal exposure, next +2, next -2
edit:
You can also find some bracketing scripts on the wiki and forum.