Thanks for the quick
I am using the camera in P mode.
I took two consecutive shots of the same scene, its pretty dim room.
1. With ISO set to 100 and Ev Step set to 1/40 I took a shot by pressing the shutter half way and let the AF lock followed by a full press to take the shot. I went to view the image and saw that the image details to be ISO 800 and shutter speed as 1/20
The image looked fairly bright as if I had taken it with auto mode.
2. With the same ISO set to 100 and Ev step set to 1/40 I took the shot this time pressing the shutter button all the way down without waiting for the AF to Lock and ended up checking the image details to show it had indeed taken a shot with the set ISO as 100 and shutter as 1/40. The image was pretty dark as expected.
I was initially confused too and double checked that I was using the camera in Auto or Program mode.
Also if it helps in any way I had tried the above test with the ISO set to auto, but the camera was still in Program mode.
Maybe I will try to set the ISO manually from the camera menu to some value. But I am not sure if thats the way I need to use CHDK ISO override. Please correct my understanding if Im wrong, irrespective of the Auto ISO I expect the CHDK setting to override it. But just to be clear I tried all the above with the camera in Program mode only.