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Aperture override on G9

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Offline koen.van.dijken

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Aperture override on G9
« on: 03 / March / 2011, 10:17:21 »
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I downloaded CHDK yesterday and did some tests with aperture override on my G9. Smallest out-of-factory aperture is F/8, with CHDK it is F/11. It does work fine, giving me greater depth-of-field which is useful in macro photography which I'm doing. What surprised me however, was that exposure times are not right. I'm in Av mode (aperture priority). Usually (without CHDK) the camera takes the right exposure times, but with CHDK it doesn't. Pictures are much too dark. Even with +2ev over exposure they are too dark. What is causing this, and how can I correct for it?

Greetings, Koen van Dijken



Edit: working with max. zoom, raw, ISO 80.
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Offline Microfunguy

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Re: Aperture override on G9
« Reply #1 on: 03 / March / 2011, 10:53:35 »

Hello and welcome.
Quote from: koen.van.dijken on 03 / March / 2011, 10:17:21
with CHDK it is F/11. It does work fine

I do not think so.
F/11 on the G9 is equivalent to F/52 on a 35mm camera !!
That reduces sharpness due to diffraction.
Look at the letter 'a' in your photos.
Far better to use the optimum wide aperture (do tests) and focus-stack the images.
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What surprised me however, was that exposure times are not right.

Use Tv (shutter speed) override to compensate.
If you close the aperture by one stop, make the shutter speed twice as long.

I am not sure if CHDK's Auto ISO option will help, the description is rather complicated :

http://chdk.wikia.com/wiki/CHDK_User_Manual#Custom_Auto_ISO


David
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Offline fudgey

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Re: Aperture override on G9
« Reply #2 on: 03 / March / 2011, 12:49:06 »
Quote from: Microfunguy on 03 / March / 2011, 10:53:35
Use Tv (shutter speed) override to compensate.

Or regular Canon ev compensation (the +- button) -- from Canon Av mode setting f/8 it's range is useful to compensate CHDK overrides up to f/16 (+- 2ev) and from scripts you can go up to +-4 ev with propcases and still have autoexposure.
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Offline Microfunguy

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Re: Aperture override on G9
« Reply #3 on: 03 / March / 2011, 13:19:05 »
Quote from: fudgey on 03 / March / 2011, 12:49:06

Or regular Canon ev compensation

As I understand it, Koen said that he tried that but it made no difference.

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Offline koen.van.dijken

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Re: Aperture override on G9
« Reply #4 on: 03 / March / 2011, 14:23:25 »
Quote from: fudgey on 03 / March / 2011, 12:49:06

Or regular Canon ev compensation

Quote from: Microfunguy on 03 / March / 2011, 13:19:05

As I understand it, Koen said that he tried that but it made no difference.




That is right: regular Canon ev compensation does not make any difference.
I noticed that the regular ev compensation is active until I half-press the shutter button. I can see the LCD getting brighter. As soon as I halfpress the shutter, it gets darker.
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Offline Microfunguy

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Re: Aperture override on G9
« Reply #5 on: 03 / March / 2011, 15:04:15 »
Check the CHDK menus in case you have Tv or ND filter override enabled.
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Offline koen.van.dijken

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Re: Aperture override on G9
« Reply #6 on: 03 / March / 2011, 15:17:08 »
Quote from: Microfunguy on 03 / March / 2011, 15:04:15
Check the CHDK menus in case you have Tv or ND filter override enabled.


The 'Value factor' for 'Override shutter speed' if Off, and 'ND filter state' is Off as well. I think this is good.

I did one more test: Auto bracketing with increasing shutter times. Increasing exposure by 1eV each time made the second picture exposed right. (aperture from f/8 to f/11, 1 stop = 1 ev).
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Offline fudgey

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Re: Aperture override on G9
« Reply #7 on: 04 / March / 2011, 13:55:52 »
Quote from: koen.van.dijken on 03 / March / 2011, 15:17:08
The 'Value factor' for 'Override shutter speed' if Off, and 'ND filter state' is Off as well. I think this is good.

It may not be the case here, but disabling (or rather, not enabling) ND override may not be enough to make the camera do what you think should happen, because Canon firmware will choose to swing in the ND filter based on its own Av choice instead of your override. In general you should probably always override ND when overriding the Av or Tv if you need autoexposure; otherwise the camera may surprise you unpleasantly.

Another simple check: did you have plenty of light? Canon's autoexposure will never choose Tv > 1 s, so if it needs a long Tv at at f/8, a positive Ev compensation will have no effect (or an effect smaller than expected). The same holds opposite in the fast Tv end for negative Ev compensation. Note that this happens even without CHDK, overrides just make this a bit more complex.
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