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Canon A1400 CHDK overrides

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Offline Crispy

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Canon A1400 CHDK overrides
« on: 10 / June / 2015, 06:39:56 »
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Hi

I’ve been doing some experimenting with my book scanner rig last night using my A1400s; thanks to waterwingz for helping me crack the problem of zoom-induced crashes.

I have a couple of other queries from the images I took last night; I’d be grateful for any advice on these issues.

The first is variable shutter speeds when taking images. The first image represents what I would call a useable exposure for post-processing. The shutter speed was 1/20.

The second image, for me, is too dark. Shutter speed was 1/80.

What is confusing me, is that all other variables with the cameras were identical. ISO 10, f/4, identical lighting conditions. So I am guessing that the variation in speed is either due to a difference between the two cameras, or, what was being photographed, eg a large white (brighter) subject, and a more text-heavy (darker) subject.

I may be answering my own question here, but is the way to ensure a uniform shutter speed to change >Disable Overrides, and go into >Override Shutter Speeds and enter the required values?

The other issue concerns focus; I have mentioned setting manual focus before. I have worked out how to disable autofocus. Will setting the subject distance in chdk override the AF Frame Mode set on the A1400? I am guessing that I’ll also need to fiddle with aperture settings?

Sorry for these long, endless enquiries. I have a basic knowledge of photography, and it can be confusing working out how chdk and the native A1400 firmware interact with each other.

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Offline waterwingz

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Re: Canon A1400 CHDK overrides
« Reply #1 on: 10 / June / 2015, 09:10:06 »
Quote from: Crispy on 10 / June / 2015, 06:39:56
The first is variable shutter speeds when taking images. The first image represents what I would call a useable exposure for post-processing. The shutter speed was 1/20. The second image, for me, is too dark. Shutter speed was 1/80. What is confusing me, is that all other variables with the cameras were identical. ISO 10, f/4, identical lighting conditions.
I assume that's a typo and you meant to say ISO 100?   

What happens when you reverse the cameras? Is one always darker than the other?

Have you verified that both cameras are setup identically in all Canon settings?

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what was being photographed, eg a large white (brighter) subject, and a more text-heavy (darker) subject.
IMHO those two subjects are not sufficiently different to justify much exposure difference.   Unless the cameras are using a very center weighted spot size for exposure.  But both those images are white in the center - it's not like one camera is pointed at the black area and the other at the white area.

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I may be answering my own question here, but is the way to ensure a uniform shutter speed to change >Disable Overrides, and go into >Override Shutter Speeds and enter the required values?
That was going to be my next suggestion.   You will need to override both the shutter speed  (Tv or time value)  and ISO setting (Sv or sensitivity value) in the CHDK Enhanced Overrides Menu.   (Make sure the Canon ISO mode for the cameras is set to Auto. Not all cameras need that but some do).

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The other issue concerns focus; I have mentioned setting manual focus before. I have worked out how to disable autofocus. Will setting the subject distance in chdk override the AF Frame Mode set on the A1400?
The A1400 is reported  ( link ) to be very well behaved when using CHDK SD overrides (i.e. MF).  So you should be able to just set a SD override in the CHDK Enhanded Photo Operations menus and go with that.  You will need to use the cameras in P mode (not AUTO) and disable Continuous AF, Tracking AF, and Servo AF (if those choices exist in the Canon shooting mode menu).   Set AF Frame mode to Center (definitely not Tracking AF).

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I am guessing that I’ll also need to fiddle with aperture settings?
Your cameras have fixed apertures so that will not be an issue. They do have an ND filter that can be inserted by the camera in very bright lighting conditions.  You can force it to stay out with the CHDK Enhanded Photo Operations menu.

Other reported changes in f-stop occur only with changes in zoom position.
« Last Edit: 10 / June / 2015, 09:17:49 by waterwingz »
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Offline Crispy

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Re: Canon A1400 CHDK overrides
« Reply #2 on: 10 / June / 2015, 09:17:34 »
Hi waterwingz, there was a setting to take the ISO down as low as 10; I thought it was strange that this was supported, but here's the grab of the metadata.

I haven't reversed the cameras, but they're identical in all settings. I'll double check this when I get home, as tiredness and inaccuracy were creeping in by the end of last night...

Thanks for all your help!
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Offline waterwingz

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Re: Canon A1400 CHDK overrides
« Reply #3 on: 10 / June / 2015, 09:22:05 »
Quote from: Crispy on 10 / June / 2015, 09:17:34
Hi waterwingz, there was a setting to take the ISO down as low as 10; I thought it was strange that this was supported, but here's the grab of the metadata.
A setting where?  In the CHDK or Canon menus?

The A1400 specs state that the available ISO settings are Auto 100, 200, 400, 800, 1600

If you found a way to get CHDK to try for ISO 10 then that's a bug.  CHDK might try for that but it can't force the camera to do what it is not physically capable of doing.

I don't know how to explain this but I can pretty much guarantee that the A1400 does not have an ISO10 setting.  No Canon P&S camera does AFAIK.

Quote from: Crispy on 10 / June / 2015, 09:17:34
I haven't reversed the cameras, but they're identical in all settings. I'll double check this when I get home, as tiredness and inaccuracy were creeping in by the end of last night...
We already know the cameras are different from your zoom problems. Checking for exposure differences when presented with identical targets seems like a logical step.  Also,  repeat the test a few times with each camera with a power off - on cycle between each shot.

« Last Edit: 10 / June / 2015, 09:25:44 by waterwingz »
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Offline Crispy

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Re: Canon A1400 CHDK overrides
« Reply #4 on: 10 / June / 2015, 09:25:10 »
Hi, that's weird; yes I was able to select ISO-10 in the chdk menu; I don't have either camera to hand at the moment, but I'll grab a pic when I get home. I remember it was displayed in the top left corner of the camera screen though.
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Offline waterwingz

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Re: Canon A1400 CHDK overrides
« Reply #5 on: 10 / June / 2015, 09:32:26 »
Quote from: Crispy on 10 / June / 2015, 09:25:10
I was able to select ISO-10 in the chdk menu;
Hmmmm .. knowing that changes things a lot!   

Not just the ISO10 setting though (which is possible to do in the CHDK menu but won't actually give you a lower setting like that - the camera will just default to ISO80 or ISO100 and ignore your setting).

What you have done is override the ISO setting while allowing the camera to pick the shutter speed.   When you do that,  the camera does not know you have changed the ISO value and it naturally assumes that the ISO value it wanted to use is the actual value used when it calculates the shutter speed to use.    The Canon firmware does not know CHDK changed the ISO setting and thus does not set the shutter speed using the CHDK ISO setting!   

You will only get the correct exposure this way by random chance.

When using any CHDK exposure override settings,  you must set all the values  ( Tv, Sv, and ND filter) to be assured of a particular exposure.   This is one of those little CHDK secrets we don't really emphasize enough IMHO.


« Last Edit: 10 / June / 2015, 09:34:01 by waterwingz »
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Offline Crispy

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Re: Canon A1400 CHDK overrides
« Reply #6 on: 10 / June / 2015, 09:41:22 »
Aha, that's very illuminating; yes it's not apparent that all this values need to be set. As I say, I don't have my camera to hand, but I remember seeing the option set the ND Filter to OUT. Are the shutter priority and aperture value settings under >Extra photo operations?
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Offline waterwingz

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Re: Canon A1400 CHDK overrides
« Reply #7 on: 10 / June / 2015, 09:42:52 »
Quote from: Crispy on 10 / June / 2015, 09:41:22
Are the shutter priority and aperture value settings under >Extra photo operations?
Yes .   But as I explained a couple posts earlier,  your camera has a fixed aperture.  You can't change it.
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Offline Crispy

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Re: Canon A1400 CHDK overrides
« Reply #8 on: 10 / June / 2015, 09:46:31 »
Cheers, I'll set all the values as suggested and see if I can get a guaranteed exposure that way.
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