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ISO override does not seem to work

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Offline rrmacha

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ISO override does not seem to work
« on: 14 / April / 2013, 09:52:38 »
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Hi

Using a Canon SX 40 with 2695 build. Somehow the ISO override does not seem to work. If I set the ISO to 50, the onscreen number in red shows 70 and the actual picture shows ISO 100.
If I set the ISO to 200, the onscreen number shows a value of ISO 200, but the picture is taken at ISO 320!

I am foxed. Can anybody help me?
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Offline waterwingz

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Re: ISO override does not seem to work
« Reply #1 on: 14 / April / 2013, 09:56:03 »
What you are seeing is the Canon "marketing" ISO value vs the CHDK "real" ISO value.   The CHDK value can be used for exposure calculations in scripts using the APEX system.  The Canon value can be used to let Canon advertise a more sensitive sensor.

See : http://chdk.wikia.com/wiki/CHDK_User_Manual#Override_ISO_value
« Last Edit: 14 / April / 2013, 09:57:48 by waterwingz »
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Ported :   A1200    SD940   G10    Powershot N    G16
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Offline lapser

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Re: ISO override does not seem to work
« Reply #2 on: 14 / April / 2013, 15:24:42 »
Wouldn't it make more sense to use "Market ISO" for the input here, and have CHDK do the conversion to "Real ISO" behind the scenes?
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EOS-M3_120f / SX50_100b / SX260_101a / G1X_100g / D20_100b
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Offline waterwingz

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Re: ISO override does not seem to work
« Reply #3 on: 14 / April / 2013, 15:40:06 »
Quote from: lapser on 14 / April / 2013, 15:24:42
Wouldn't it make more sense to use "Market ISO" for the input here, and have CHDK do the conversion to "Real ISO" behind the scenes?
After more than six years of forum comments on this subject, I'm not going to touch that one with a ten foot pole ...
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Ported :   A1200    SD940   G10    Powershot N    G16
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Offline lapser

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Re: ISO override does not seem to work
« Reply #4 on: 14 / April / 2013, 16:15:28 »
Quote from: waterwingz on 14 / April / 2013, 15:40:06
After more than six years of forum comments on this subject
Yes, I'm realizing that there's no consensus on a lot of things in CHDK. But there's an option to display real or market ISO, so shouldn't there at least be an option to input real or market? (You could call this a "5 Foot Pole")
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EOS-M3_120f / SX50_100b / SX260_101a / G1X_100g / D20_100b
https://www.youtube.com/user/DrLapser/videos
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Offline reyalp

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Re: ISO override does not seem to work
« Reply #5 on: 14 / April / 2013, 16:46:50 »
Quote from: lapser on 14 / April / 2013, 15:24:42
Wouldn't it make more sense to use "Market ISO" for the input here, and have CHDK do the conversion to "Real ISO" behind the scenes?
Yes.

All we need is someone to come up with a coherent, consistent way to do it that doesn't require special code for each camera.
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Offline rrmacha

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Re: ISO override does not seem to work
« Reply #6 on: 15 / April / 2013, 00:03:17 »
Thanks for the reply Waterwingz. Sincerely Appreciate it.
When I take a picture with an override of ISO 80, does the camera actually get set to a lower ISO? The reason I ask is that I am not able to see any difference in the picture taken with a lower ISO. All other settings of Aperture and time are exactly the same only the ISO has been changed.  In my older SX 100, I could manually set the camera to ISO 80 and there is a distinct difference between pictures taken at ISO 100 and ISO 80.
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Offline waterwingz

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Re: ISO override does not seem to work
« Reply #7 on: 15 / April / 2013, 08:42:00 »
Quote from: rrmacha on 15 / April / 2013, 00:03:17
When I take a picture with an override of ISO 80, does the camera actually get set to a lower ISO?
It varies somewhat between camera models but in general, CHDK cannot force the Canon firmware to go below its lower ISO limit. This limit can be slightly lower than the lowest dial setting on some cameras - you just have to experiment and see for yourself.
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Ported :   A1200    SD940   G10    Powershot N    G16
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Offline lapser

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Re: ISO override does not seem to work
« Reply #8 on: 15 / April / 2013, 15:10:04 »
Quote from: reyalp on 14 / April / 2013, 16:46:50
All we need is someone to come up with a coherent, consistent way to do it that doesn't require special code for each camera.

I need to research it a little more for specific cameras, but I think all you need is the difference in sv96 between "real" and "market". In my cameras, it is 59. Using my new functions, sv96_to_iso() and its inverse, here's how I do it in my script:

Code: (lua) [Select]
press_half() -- includes wait for get_shooting()
sv=get_sv96() -- real
svdiff=iso_to_sv96(get_iso_market())-sv  -- real to market difference
svmin=iso_to_sv96(s)-svdiff -- min sv ISO 100 market default
svmax=iso_to_sv96(z)-svdiff -- s and z are script input parameters

-- to output market ISO from sv96, you do this:
sv=411 -- test
print(sv96_to_iso(sv+svdiff)) -- prints market ISO
It seems to work if I set the camera to ISO 100 before starting the script. If I set it to ISO 3600, the real sv96 is off, and the svdiff isn't 59. I think this may be an error in the current get_iso_market or get_sv96().

If sv96_to_iso(sv)==iso
then
sv96_to_iso(sv+96)==iso*2

This should be true for real or market ISO, shouldn't it?
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EOS-M3_120f / SX50_100b / SX260_101a / G1X_100g / D20_100b
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Offline reyalp

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Re: ISO override does not seem to work
« Reply #9 on: 16 / April / 2013, 01:06:58 »
Quote from: lapser on 15 / April / 2013, 15:10:04
In my cameras, it is 59.
For every ISO setting? That is, if you go through the canon UI and set each of the available ISO values, is the corresponding value in the "real" propcase offset by 59? This definitely does not apply to all cameras.

Since  http://chdk.setepontos.com/index.php?topic=8613.60 for some further discussion and observations.
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