I finally got around to testing this with the latest release (a650-100d-0.9.7-725-full).
I did two tests and each had two sections. The first section was using bracketing without overriding the shutter speed, and the second used bracketing and shutter override.
For the first test I set the aperture to 8, the shutter speed to 1/2000 (the camera's normal max), bracketing to 1 EV, and set the camera for 4 shots. The expected, actual, and JPG EXIF shutter speeds are all identical. Here are the results:
Frame # | 1 | 2 | 3 | 4 |
Shutter Speed | 1/2000 | 1/1000 | 1/500 | 1/250 |
Next I left everything else the same and set the shutter override to 1/50000. This time, the expected, actual, and exif speeds did not match. I determined the actual speeds by comparing the images to the previous set. With bracketing at 1 EV, it was easy to see which images were at the same shutter speed.
Frame # | 1 | 2 | 3 | 4 |
Expected Speed | 1/50000 | 1/25000 | 1/12500 | 1/6000 |
Actual Speed | 1/50000 | 1/1000 | 1/500 | 1/250 |
JPG EXIF Speed | 1/2000 | 1/1000 | 1/500 | 1/250 |
I started to suspect that while the shutter override controlled the speed of the very first frame, for the speed of subsequent frames, the CHDK bracketing program didn't look at the override speed and instead looked at whatever speed the camera was set to by its own, non CHDK controls (as in the speed it would do without the shutter override). To test this, I did another series starting at a much lower shutter speed. First are the results without using shutter override. Again, the expected, actual, and exif speeds were the same.
Frame # | 1 | 2 | 3 | 4 |
Shutter Speed | 1/200 | 1/100 | 1/50 | 1/25 |
Next I again left everything else the same, including the cameras own shutter speed set to 1/200. I then enabled shutter override and set it to 1/50000.
Frame # | 1 | 2 | 3 | 4 |
Expected Speed | 1/50000 | 1/25000 | 1/12500 | 1/6000 |
Actual Speed | 1/50000 | 1/100 | 1/50 | 1/25 |
JPG EXIF Speed | 1/2000 | 1/100 | 1/50 | 1/25 |
This data appears to confirm my suspicions. For this last test, the first frame used the shutter override speed like it was supposed to, but when bracketing took over for the next three frames, it didn't look at the override speed, it instead looked at the camera's own shutter speed, which was still set to 1/200. In seeing that, it assumed the first frame must have been at that speed, so the next three were at 1/100, 1/50, and 1/25.
Also, the JPG EXIF data continues to be wrong whenever an override is set outside the cameras normal boundaries. While the actual speed of the first frame was 1/50000, the camera refused to write anything higher than 1/2000. This same problem occurs with exposures longer the camera's maximum of 15 seconds. If I override it to 65 seconds, the exif data will still say 15. However, whatever causes the exif data to be wrong seems to be different from the bracketing problem. If they both looked at the same thing, I imagine the actual speeds of this last round would have been 1/50000, 1/1000, 1/500, 1/250 instead of 1/50000, 1/100, 1/50, 1/25.