30D 1/3-stop ISOs - FAKE!

Started May 7, 2006 | Discussions thread
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John Sheehy Forum Pro • Posts: 21,042
30D 1/3-stop ISOs - FAKE!

I finally have a new version of IRIS that reads 30D RAW files properly, and just as I suspected, the extra ISOs on the 30D are arithmetic exposure indexes; they are not analog amplifications. This means that these "in-between" ISOs have slightly inferior RAW data with comb-like histograms. The file I am examining is ISO 320. Every third (or fourth) value in the RAW histogram is twice as populated as the in-between ones.

What the camera is doing for this ISO 320 file, apparently, is over-exposing ISO 400 by 1/3 stop, and then after digitizing the RAW data, compresses it down to about 80% as many values. You could do this better by just exposing to the right at ISO 400, without getting any of the posterization or loss of 1/3 stop of highlights that the camera does when doing its inferior "ISO 320".

This is the kind of worthless stuff Canon gives us instead of RAW RGB histograms, or auto-ISO manual mode.

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John

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