I have a question for those familiar with trying to use fixed shutter speeds.. I've got CHDK loaded on my SD450 (Ixus 55), and I've got a intervalometer script running that allows me to fix the shutter speed. I tested it out, and it works fine, fills up the memory card, shutter speed stays accurate the whole time. My problem is that the aperture value doesn't change based on brightness (in this instance, it stayed at f/2.8 ). If this was a camera in an actual Tv mode, the shutter speed would be fixed, while the aperture size would change based on lighting conditions. My script does nothing except set the ISO at 100 and set Tv to whatever I tell it to (in this example, it was 1/15").
I think I've read that these small compact cameras don't have a physical aperture? Do I need to perhaps set ISO to auto, instead of locking it at 100? I haven't looked too much into this, as I assumed locking the shutter speed would make the camera act like it was in a true Tv mode... Is there a Digic II property I can set_prop to make this more like Tv? Any other scripting ideas, or examples I might be able to look at?
An example from my fridge (brightest light I could find easily... pardon the focus problems)
Canon 20D (Actual Tv mode) 1/15" f/10
Canon SD450 (Shutter set at 1/15") f/2.8