Can't get a true Tv mode - aperture stuck at f/2.8

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Can't get a true Tv mode - aperture stuck at f/2.8
« on: 12 / May / 2010, 20:15:36 »
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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




Re: Can't get a true Tv mode - aperture stuck at f/2.8
« Reply #1 on: 12 / May / 2010, 20:37:40 »
I think I've answered this, but would appreciate some confirmation. There is no physical iris/aperture, we get an ND filter on these point & shoots instead. It looks like turning the ND filer on (in) sets it to f/8, but this is NOT reflected in the exif data, which is why I was confused..

Here's the same (similar) shot with ND filter in.



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Offline fe50

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Re: Can't get a true Tv mode - aperture stuck at f/2.8
« Reply #2 on: 14 / May / 2010, 08:29:26 »
I think I've answered this, but would appreciate some confirmation. There is no physical iris/aperture, we get an ND filter on these point & shoots instead. It looks like turning the ND filer on (in) sets it to f/8, but this is NOT reflected in the exif data, which is why I was confused..
Yes, correct.
With the ND filter disabled you'll get the smallest F number (e.g. F 2.8 ), with enabled ND filter the highest, e.g. F 8.0
The "aperture" will also change mechanically with the zoom.

Re: Can't get a true Tv mode - aperture stuck at f/2.8
« Reply #3 on: 14 / May / 2010, 08:33:24 »
Thanks fe50! My concern was getting the dof associated with f/8, not really the amount of light. I see though, with no real aperture, the dof remains constant. Oh well, not CHDK's fault!! :) I'm still impressed with everything I can do with a $30 camera off of ebay. :D

Thanks again!


 

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