The Ixus 860IS doesn't have an iris, it only has a neutral density filter which can only produce settings of f/2.8 and f/8. There is a CHDK setting for ND filter, but it doesn't seem to work (yet). For my purposes, I need a high shutter speed around 1/800 to 1/1000. When I tried setting the shutter speed in CHDK, it didn't automatically adjust the ND filter to f/2.8, so my images were very underexposed. I have found that setting EV +1 1/3 or greater so will force the ND filter to select f/2.8.
Hopefully the CHDK ND filter settings will be functional soon.