ISO acdcuracy test

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Hello CHDK forum
Thank you for CHDK!!!
I have been doing some astrophotography with my SX230 v1.01B.
I was uncertain about what my actual ISO was so I shot some brackets of a grey card and read the Luminosity in RawTherapee.
I also ran Auto CHDK to EXIF v1.3.

First I short exposures in native Canon mode at ISO 100 (Lum 6), 200 (L. 25), 400 (L. 31), 800 L. 45), 1600 (L. 61), & 3200 (L. 82). Luminosity values were found by mousing over the Grey Card area of images in RawTherapee. As you can see, the EXIF ISO values and Luminosity values (while not perfect) seem to be somewhat consistant.

Now on to CHDK ISO override. I set the ISO in "Override ISO Value" with the "Value Factor" set to 100. I shot exposures at:
ISO override 1600 (EXIF ISO 2415, Luminosity 1)
ISO override 3200 (EXIF ISO 4829, L. 7)
ISO override 6400 (EXIF ISO 9659, L. 4 thru

ISO override 12,800 (EXIF ISO 2933, L. 4-7)
ISO override 25,600 (EXIF ISO 5867, L. 4-7)
ISO override 51,200 (EXIF ISO 100, L. too dark)
ISO override 80,000 (EXIF ISO 5614, L. 3-6)
OK, so my testing procedures are not perfect (especially in reading the Luminosity values). I guess I just wanted to share my findings with the forum. If anybody has a better way to do things then I would appreciate hearing.
Clear Skies
apfrsscf

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