Is Custom Auto ISO working for the A640 under 1.2?

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Is Custom Auto ISO working for the A640 under 1.2?
« on: 02 / October / 2013, 18:34:28 »
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Can someone confirm that this works?  I'm in touch with a guy who says the "AutoISO" notice appears on the non-ALT screen, so it clearly is enabled, but it doesn't do anything.  On my A590 running 1.1, the custom maximum shutter speed setting is maintained unless there still isn't enough exposure after the aperture is wide open and the maximum ISO value is reached.  He says he's getting slower shutter speeds even when there's still room on the other parameters, which shouldn't be happening.

So for example, he has the maximum shutter speed set to 1/1000, max ISO to 320.  But he's getting pictures at 1/250 second, ISO at 300, and aperture at 3.5-4.0.  I think ISO should be 320, aperture at 2.8 (the max for this camera), and only then the shutter speed reduced from 1/1000.

Can anyone with an A640 confirm that this is working for him under 1.2?  Can anyone suggest what my guy may have set wrong?

Thanks for any help.

Re: Is Custom Auto ISO working for the A640 under 1.2?
« Reply #1 on: 02 / October / 2013, 18:45:39 »
I should have said - he's shooting in Program Mode, with ISO set to Auto.  Same for me.

Re: Is Custom Auto ISO working for the A640 under 1.2?
« Reply #2 on: 02 / October / 2013, 20:44:49 »
I've taken a few looks at the autoiso code - mostly to try and provide a description for the CHDK User Manual. 

http://chdk.wikia.com/wiki/CHDK_1.2.0_User_Manual#Custom_Auto_ISO

The little bit of testing I did on my A1200 showed that the code worked as described.  I think I know what might be a causing what you are seeing,  but I'm not completely sure.

One thing that stands out when I read the code is that it seems to ignore the camera's aperature setting.  Many Canon P&S cameras do not have an aperature that can be changed (other than the change that results from changing the zoom position).  On cameras with adjustable iris positions,  the autoiso code lets the camera pick the aperature and then adjusts the resulting shutter speeds and ISO values according to its built-in rules.

As your camera and your friends have variable aperatures,  that might be causing what you are seeing?
Ported :   A1200    SD940   G10    Powershot N    G16

Re: Is Custom Auto ISO working for the A640 under 1.2?
« Reply #3 on: 02 / October / 2013, 21:21:03 »
Well, the thing is, it works properly on my camera (v1.1) , but not on his (v1.2).  Also, your description isn't quite the way it works on my camera.  It does let the camera choose the aperture (it likes f4), but only if the shot can be properly exposed subject to the Custom ISO shutter speed and ISO settings.  But if more light it needed, it will open the aperture all the way before it starts reducing the shutter speed.



Re: Is Custom Auto ISO working for the A640 under 1.2?
« Reply #4 on: 02 / October / 2013, 22:21:47 »
Well, the thing is, it works properly on my camera (v1.1) , but not on his (v1.2).
While there was some rearrangement of the source files related to autoiso between v1.1 and v1.2,  and the fixes for real vs market ISO touched on it a bit,  the autoiso algorithm did not change.   I'm not challenging what you are reporting,  just suggesting that the code might be doing different things under what appears to be similiar circumstances.

It would be very interesting to see how your camera works with 1.2 or your friends with 1.1 if you have time to do some testing?

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Also, your description isn't quite the way it works on my camera.  It does let the camera choose the aperture (it likes f4), but only if the shot can be properly exposed subject to the Custom ISO shutter speed and ISO settings.  But if more light it needed, it will open the aperture all the way before it starts reducing the shutter speed.
I understand that is what you see happening.  But when I look at the code,  it never attempts to adjust aperture settings directly.  There is just no code there doing that.  This sounds more like some ill-defined interaction between Canon's automatic exposure control and the autoiso function.

Sorry if this isn't helping.  In early versions of the new CHDK 1.2 Manual,  that code was documented as "Seems to work mostly by magic".    To some extent,  that might still be the case.   I don't think anyone is in a big hurry to tackle this project,  but a look at how the Av setting is used might be a good place to start if someone was.


Ported :   A1200    SD940   G10    Powershot N    G16

Re: Is Custom Auto ISO working for the A640 under 1.2?
« Reply #5 on: 03 / October / 2013, 00:11:30 »
Ok, I'll try 1.2 on my A590 tomorrow and see how it works.

But can you confirm that aside from any aperture control, the code you're looking at does the following:

If more exposure is needed, it will first increase ISO to the limit set in the Custom Auto ISO menu before slowing the shutter speed below the Maximum setting in that same menu.

That appears to be how it behaves on my camera.  In fact, that's the whole reason why I use Custom Auto ISO so as to provide a strong preference for a fast shutter speed, for KAP, but still provide flexibility that Tv doesn't.


Re: Is Custom Auto ISO working for the A640 under 1.2?
« Reply #6 on: 03 / October / 2013, 11:57:59 »
But can you confirm that aside from any aperture control, the code you're looking at does the following:
If more exposure is needed, it will first increase ISO to the limit set in the Custom Auto ISO menu before slowing the shutter speed below the Maximum setting in that same menu.
That's not really how the code works but I think it gives you the same effect.   

What the code does is calculate the ISO setting needed when the shutter speed is set to the minimum value specified in the CHDK menu.  If that ISO value is above the maximum specified in the CHDK menu then it limits the ISO setting to that maximum.  Once it has picked an ISO setting,  it recalculates the needed shutter speed for correct exposure (which can go below the minimum shutter speed specified in the menu if necessary).

So it will essentially only lower the shutter speed below the specified minimum value if the ISO value has reached its upper limit.  Which is I believe what you want?
Ported :   A1200    SD940   G10    Powershot N    G16

Re: Is Custom Auto ISO working for the A640 under 1.2?
« Reply #7 on: 03 / October / 2013, 14:34:30 »
Yes that is at least part of what I want, and I installed 1.2 on another card, and found that on my A590 it does behave that way, as it does on 1.1.  However, on the other guy's A640, he is getting slower shutter speeds when the ISO is not maxed out.  It's possible this results from his max ISO setting of 320, which may not exist on his camera, or perhaps the difference between "market" and "real" something.  He said most of the pictures were "around ISO 300" when the shutter speed was 1/250 or so (with the setting being 1/1000 minimum).

So I suspect it's working more or less on his camera, although not as I told him it would.  But the bigger issue involves the aperture question, as well as what happens in Program mode.

You said after it sets the ISO it recalculates the correct shutter speed.  But what if you're in Program mode?  In P mode with ISO set to Auto, the camera will normally want to control all three factors.  But your code has now set the ISO and shutter speed.  Does the camera accept those fixed values and calculate the aperture?  Or is your code running AFTER P mode has done it's calculations, so that it just accepts the what the camera's P mode has chosen for aperture?

My bias in this is what I need to have for KAP.  The biggest problem is that the camera is constantly moving, so a fast shutter speed is the most important thing.  At the same time, in low light sometimes you really do need to slow down the shutter just to get the shot at all.  So the Custom Auto ISO function is really useful.  But the problem is that it lets the shutter slow down even though the aperture hasn't been maxed.  Would it be possible to change the code to include aperture control, at least in P mode?

So after the last step you described when the "final" shutter speed is determined, if that speed is slower than the target setting, but the aperture is not wide open (for those cameras that have an aperture), it would open up the aperture enough to get back to the target speed, or until it's wide open.



Re: Is Custom Auto ISO working for the A640 under 1.2?
« Reply #8 on: 03 / October / 2013, 15:29:46 »
Does the camera accept those fixed values and calculate the aperture?  Or is your code running AFTER P mode has done it's calculations, so that it just accepts the what the camera's P mode has chosen for aperture?
I believe its the latter.    The exposure still comes out correctly but the choice of f-stop is not done by the AutoISO code.

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So after the last step you described when the "final" shutter speed is determined, if that speed is slower than the target setting, but the aperture is not wide open (for those cameras that have an aperture), it would open up the aperture enough to get back to the target speed, or until it's wide open.
On most P&S cameras,  the depth of field is pretty good even at the widest lens opening.  Which leads me to wonder about the value of an adjustable iris - especially given the limited ranges of f-stops on the P&S cameras that have any adjustment at all.  I suppose stopping the lens way down ( which also tends to negatively affect sharpness ) might let you use lower shutter speeds in circumstances where you want some blur,  but that's a pretty specialized example.

So given all that,  what I'm thinking is that when AutoISO is active, maybe the aperature should just be locked at its widest opening?


Ported :   A1200    SD940   G10    Powershot N    G16

Re: Is Custom Auto ISO working for the A640 under 1.2?
« Reply #9 on: 03 / October / 2013, 17:16:19 »
Depth of field wouldn't be the issue for what I'm using CHDK for because most of the subjects are at near infinity.  But sharpness could be.  I assume that like most lenses, there's a sweet spot in the middle of the aperture range where things are sharpest.  So if the picture can be taken at that aperture, I'd like to do that, but only if the shutter speed can stay fast as well.

The other potential problem with shooting at a fixed aperture of f2.6 is that on a bright sunny day, it may be impossible not to overexpose.


 

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