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SX40: AutoISO above ISO800?

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Offline longtalker

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SX40: AutoISO above ISO800?
« on: 14 / March / 2012, 07:48:18 »
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CHDK is great for choosing the lowest ISO for which your desired minimum shooting speed (e.g. 1/30) is still met - a feature that otherwise, to my knowledge, would not be available on many Powershots including the SX40. However, in choosing the range of the overriden AutoISO, although one can set the Minimum AutoISO to "real" values (e.g. 10) that are well below the camera's minimum "market" ISO (100), the Maximum AutoISO seems to only go up to 800, although the camera supports up to 3200.

Therefore it seems that, in low-light conditions where a tripod is not available, you don't have the option of shooting at the ISO value (which may need to be 1600 or 3200) that gives you the minimum shooting speed, and you're limited to ISO800, which may mean underexposure of the scene.

I was therefore wondering whether it's likely that new builds of CHDK will be released for the SX40 that will perhaps extend the AutoISO range so as to cover (after correcting for the difference between real/market ISO values) the whole range of ISO supported by the camera?

In case an update isn't likely, I suspect it would not be difficult to change the code myself so as to extend up to AutoISO=3200, however I'm not sure if it would be safe to use this modified version on the camera, as I'm guessing CHDK devs must have probably had a good reason why they limited the upper part of the AutoISO range in the first place...?

Thanks in advance for any replies!
« Last Edit: 14 / March / 2012, 11:42:36 by longtalker »

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Offline funnel

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Re: SX40: AutoISO above ISO800?
« Reply #1 on: 14 / March / 2012, 14:59:47 »
If you wanna do it by yourself you need to edit this line in core/gui.c

Code: [Select]
MENU_ITEM(0x5f,LANG_MENU_AUTOISO_MAX_ISO_AUTO, MENUITEM_INT|MENUITEM_F_UNSIGNED|MENUITEM_F_MINMAX, &conf.autoiso_max_iso_auto, MENU_MINMAX(10, 80) ),
the 80 in MENU_MINMAX is the max iso 800. Change it to a higher number. A value of 200 (iso 2000 real) is close to canons ISO 3200.
« Last Edit: 14 / March / 2012, 15:04:52 by funnel »

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Offline longtalker

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Re: SX40: AutoISO above ISO800?
« Reply #2 on: 14 / March / 2012, 16:06:59 »
Many thanks for that! Few more questions:

(0) Really silly one, but where exactly is the gui.c file? I don't have any CORE folder under \CHDK, only BOOKS, CURVES, etc

(1) If I make this change, do I then need to re-compile any source code, or will saving the .c file do?

(2) How risky would it be to make this change, given that, as I mentioned before, I'm sure the CHDK developers must have had a reason to set that seemingly arbitrary upper limit for the AutoISO? I know CHDK can actually harm the camera if it instructs it to do something that it wasn't designed to do from a hardware point of view, although I've no idea whether forcing a certain AutoISO would constitute such an instruction...

(3) Is there a calculator that can convert between the "real" ISO values and the ones displayed by the camera (the "market" values as they are referred to in the CHDK manual)?

(4) I know that CHDK builds are released nightly, but since the changelog document is not available (https://chdk.kernreaktor.org/chdk-changelog.txt seems to always be down), it's hard to tell whether new features are being implemented (or old features fixed) in every new build. If I knew that people are still working at, for instance, the SX40 version, then I'd like to contribute suggestions/bug reports etc.

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Offline reyalp

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Re: SX40: AutoISO above ISO800?
« Reply #3 on: 14 / March / 2012, 16:33:28 »
Many thanks for that! Few more questions:

(0) Really silly one, but where exactly is the gui.c file? I don't have any CORE folder under \CHDK, only BOOKS, CURVES, etc
It's part of the source code. If you don't have some programming experience, this may not be for you.
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(1) If I make this change, do I then need to re-compile any source code, or will saving the .c file do?
You would need to build. You can use chdkshell for that.
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(2) How risky would it be to make this change, given that, as I mentioned before, I'm sure the CHDK developers must have had a reason to set that seemingly arbitrary upper limit for the AutoISO?
Hard to say why that limit was originally there, but it probably dates from a time when the cameras didn't go higher than 800. Allowing higher than the camera is capable of would result in incorrect exposures.
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I know CHDK can actually harm the camera if it instructs it to do something that it wasn't designed to do from a hardware point of view,
As far as we know, no one has actually managed to do this, although it could have happened in some way that was not conclusively attributable to CHDK.
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although I've no idea whether forcing a certain AutoISO would constitute such an instruction...
I'd rate this as unlikely. People have tried absurdly high ISO overrides with no ill effect many times, and this wouldn't be any different.
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(3) Is there a calculator that can convert between the "real" ISO values and the ones displayed by the camera (the "market" values as they are referred to in the CHDK manual)?
No. The difference is not well specified, but you you can easily find out what it is on your camera by examining the relevant propcases while you set different ISO values in the Canon UI.
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(4) I know that CHDK builds are released nightly, but since the changelog document is not available (https://chdk.kernreaktor.org/chdk-changelog.txt seems to always be down)
To see changes in the "stable" release version (found on the main autobuild) see here: http://trac.assembla.com/chdk/log/branches/release-1_0

All the changes on every branch can be seen here: http://trac.assembla.com/chdk/log/
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If I knew that people are still working at, for instance, the SX40 version, then I'd like to contribute suggestions/bug reports etc.
There is no point point where work is stopped on a particular port. Changes are made when they are needed and a developer knows what change to make.
Don't forget what the H stands for.


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Offline longtalker

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Re: SX40: AutoISO above ISO800?
« Reply #4 on: 14 / March / 2012, 17:03:01 »
Hi reyalp, thanks very much indeed for your answers!

I have *some* programming experience, however if I had the option I'd prefer to leave it to someone who better knows what they're doing. Since this change (increasing the max AutoISO to a Canon equivalent of 3200) would be a quite simple one to make and would benefit all SX40 users, I wonder whether philmoz (who seems to be the main SX40 contributor) would consider including it in the next update?...

Thanks again, I very much appreciate the help!

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Offline blackhole

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Re: SX40: AutoISO above ISO800?
« Reply #5 on: 14 / March / 2012, 17:31:04 »
If you move your ISO to 3200 that does not mean that RAW will be different than the ISO 1600 or less, the difference can only be seen on the JPG what was achieved with a treatment program, not the hardware.

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Offline longtalker

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Re: SX40: AutoISO above ISO800?
« Reply #6 on: 14 / March / 2012, 19:46:35 »
One more thing, in addition to the arbitrary AutoISO 800 limit: I noticed that, in very low light conditions, the camera (or rather: CHDK) doesn't even make use of the maximum available autoISO valuel: it never selects an AutoISO higher than 400, even though the Max AutoISO is set to 800. This is unexpected, as if you go from daylight and progressively decrease available light, the selected AutoISO keeps increasing, so as to meet the set minimum exposure, however it stops at ISO 400!

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Offline blackhole

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Re: SX40: AutoISO above ISO800?
« Reply #7 on: 15 / March / 2012, 03:56:41 »
Here is a version where you can set MaxAuto ISO to 2000.


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Offline longtalker

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Re: SX40: AutoISO above ISO800?
« Reply #8 on: 15 / March / 2012, 05:13:23 »
Many thanks blackhole, will give this a try and let you know!

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Offline longtalker

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Re: SX40: AutoISO above ISO800?
« Reply #9 on: 15 / March / 2012, 20:25:29 »
There seems to be a problem with this build. After setting Max AUto Iso to 2000, in HQ mode the camera always selects an exposure of 1", at ISO800, even in bright light. Setting it back to 800 as I had it in the old build doesn't correct the problem. Was increasing the max auto ISO from 800 to 2000 the only change that you made?

By the way, it would be great if the step in varying the ISO value would be in multiple of 10s... takes about 30 sec or so of holding the right button down to go from 800 to 2000 :)

 

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