I have been exploring the way the ISO override (in AllBest's v 12 and v15) works on the A610 and S3, using the M (manual) mode setting. In the course of these tests I have come across some behaviour that I have not seen mentioned in other posts.
In certain circumstances the overriden ISO value applies, but in other cases the picture is taken with the ISO value set on the camera, and in other cases the ISO value appears to be the highest that the camera is capable of using (that is, it is not the ISO value set on the camera, nor the overridden ISO value specified in the CHDK menu). This commonly happens when the shutter button is given a 'full press', instead of first giving a 'half press', before completing the button-press.
There is a pattern to this behaviour, but instead of trying to remember the pattern, the safest way to be sure of using the specified (overridden) ISO seems to be to "load" the override ISO into the camera.
For the S3 I found that the following procedure "loads" the overridden ISO value:
1. 'Half press' the shutter button for about half-a-second. The histogram display shows that adjustments are being made.
2. Let the shutter button up again, without making an exposure.
3. 'Half press' the shutter button again for about half-a-second. The histogram display again shows that an adjustment is being made.
After this has been done, the camera will use the overriden ISO value, even if the shutter button is given a rapid 'full press'.
For the A610, just giving the shutter button a half-second 'half press' appears to "load" the overridden ISO value.
Maybe there is an easier way - has anyone had similar experiences with these cameras (or other cameras)?
By the way, the method I used to check the ISO value was to take pictures of a dark blue piece of cardboard in indirect skylight. The histogram for this set-up is a single narrow peak only about 10% of the width of the whole histogram. By changing the exposure settings the peak can be placed anywhere on the histogram. Judging from the shifts in the peak position when the overridden ISO value is changed (with fixed exposure time and aperture) the ISO range for the A610 appears to be from about 30 to 600 (the marked values are 50 to 500). For the S3, the available ISO values appear to vary from about 50 to about 1000 (marked values are 80 to 800).