Override 1 s exposure maximum in AV mode on G9? - General Help and Assistance on using CHDK stable releases - CHDK Forum
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Override 1 s exposure maximum in AV mode on G9?

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Offline bouvin

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Override 1 s exposure maximum in AV mode on G9?
« on: 28 / May / 2009, 19:09:09 »
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Hi

I'm using CHDK with a G9. One of my uses for my G9 is to shoot large panoramas using a Gigapan Imager, which I since stitch together with AutoPanoGiga.  I shoot my panos in AV mode, as a fixed aperture is critical to ensure good stitching. In low light situations, the exposure naturally becomes longer (especially as an aperture of f/8 can be quite desirable to ensure good field-of-depth), and here I run against a Canon imposed limitation, as the exposure time in AV mode cannot exceed 1 second. I can of course hardwire aperture and exposure in manual mode, but that is not feasible, as the correct exposure can vary wildly across a scene (consisting of hundreds of pictures).

So, have I overlooked the way of overriding the 1 second limit using CHDK, or can it be accomplished in some way through uBasic or Lua?  I'm perfectly fine with the existing AV mode (indeed automatic exposure is necessary, as some of my panos consist of 500-600 images), but I would like access to longer exposures.


Cheers
          Niels Olof
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Offline Microfunguy

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Re: Override 1 s exposure maximum in AV mode on G9?
« Reply #1 on: 29 / May / 2009, 09:23:44 »
I do not use the standard CHDK but there must be a menu item where you can override exposure.
Have you tried that ?

However, if I understand you correctly, you actually want an automatic exposure mode where a long exposure time will be set if required.

Is that correct ?

A CHDK override does not provide automatic exposure.


David
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Offline fudgey

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Re: Override 1 s exposure maximum in AV mode on G9?
« Reply #2 on: 29 / May / 2009, 12:34:44 »
Canon autoexposure never selects Tv > 1 second. CHDK doesn't change that.

But I do have a solution for this particular setup. Write a script (Lua or uBasic) that does the following (in Av mode with manual ISO):

1) set aperture wide open
2) half shoot and wait for autoexposure
3) read exposure time (tv96). You now know Canon's choice for Tv assuming wide aperture, i.e. you know how dark it is according to your camera.
4) release half shoot
5) set aperture to F/8
6) adjust tv96 as much as you just adjusted the aperture av96
7) shoot for real

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Offline bouvin

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Re: Override 1 s exposure maximum in AV mode on G9?
« Reply #3 on: 30 / May / 2009, 17:39:18 »
Excellent advice, and a good starting point for getting started writing Lua.  Thank you for your help.

/Niels Olof
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