CHDK has lower ISO limit than camera's own limit (on IXUS 870 / Powershot SD880)

Lebenita

« on: 22 / August / 2009, 15:33:29 »

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I recently tested CHDK's ISO limits on my IXUS 870 (Powershot SD880).
The lower limit seems to be about the same as the camera's own limit (around ISO 80, reported by CHDK as "real" ISO 53).
The upper limit, however, is around ISO 800 ("real" ISO as reported by CHDK: 530), as opposed to ISO 1600 (the camera's regular upper limit) or 3200 (which is available as a scene mode).

I can set sensitivities higher than ISO 800 in CHDK (in the ISO override setting or in the custom auto ISO setting), but the resulting pictures are underexposed, because actual sensitivity in fact stays fixed somewhere in the region of ISO 800. This is not the case when I use the camera's own 1600 or 3200 setting.

Does this ring a bell with anyone? Could it be a porting problem? Or a general CHDK limitation?

It would really be nice if I could use CHDK's custom auto ISO function to extend the HI ISO limit from 800 to 1600.

Thanks for any thoughts,
Lebenita

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reyalp

11387

Re: CHDK has lower ISO limit than camera's own limit (on IXUS 870 / Powershot SD880)

« Reply #1 on: 22 / August / 2009, 17:59:13 »

ISTR on most canon cameras, ISO above 800 is actually done in software in the develop process, rather than turning up sensor gain (some cameras also have a high ISO mode, which appears to use binning)

Maybe CHDKs auto ISO should include these, but it requires using something other than the normal override method.

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Don't forget what the H stands for.

Lebenita

IXUS 870 / Powershot SD880 has hardware sensitivity limit at ISO 800

« Reply #2 on: 25 / August / 2009, 06:13:22 »

Thanks, Reyalp, that was it. Above ISO 800, raw files look identical regardless of the sensitivity displayed on screen. The JPEGs are simply brightened up by the camera (which explains why they look so bad). This brightening-up for some reason does not work when using CHDK's custom auto ISO, leading to "underexposed" JPEGs, but at anything above ISO 400 (or even 200), you are better off processing the raw files anyway.

At ISO 3200, the raw file produces gibberish when processed in DCRAW, probably because the pixel binning you've mentioned changes the file structure.

Thanks again,
Endre

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