ISOless sensors, read noise and photography - many questions!

Started Oct 3, 2013 | Discussions thread
Steen Bay Veteran Member • Posts: 7,300
Highlight headroom

boardsy wrote:

It's not obvious (or embarrassing not to have known) that many (most?) high ISO values in many (most?) cameras are not a useful sensor sensitivity gain, but simply a tag in a RAW file to tell the JPG or RAW software where to set brightness, and stretching the actual exposure (aperture & shutter speed) over the available brightness levels, so "high ISO noise" is in fact low exposure noise, exposed (!) by raising brightness levels!! I've only just discovered all this, in a personal Copernican ISO revolution!

Would be ideal if the ISO (on so-called 'ISO-less' cameras with a 'flat' read noise) just was a tag in the RAW file, but it isn't. The 'stretching' is (normally) done before the RAW file is saved on the memory card. If it just was a tag in the RAW file, then it wouldn't be necessary to shoot 'ISO-less' and get dark JPEGs. Then you could just use auto ISO and get JPEGs with the 'correct' brightness, while the RAWs automatically would have e.g. 4 stops more 'headroom' (above metered middle grey) at ISO 1600 than at ISO 100.

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