Exposure basics, lesson two point one (& ISO)

Started Mar 19, 2013 | Discussions thread
Detail Man
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Re: Exposure basics, lesson two point one (& ISO)
In reply to Timur Born, Mar 20, 2013
There is an assumption that the image is in sRGB with a standard 2.2 Gamma (as modified by sRGB) but we all know that different manufacturers adopt different tone curves for their default jpegs - olympus for instance are particularly contrasty - so, you can set the 18% grey point,

Since sRGB doesn't use an exact Gamma of 2.2, but a pushes dark shadows slightly compared to a pure 2.2 curve I just ask myself whether 18% gray is on the 2.2 part of the curve or still on the pushed part?

It does not push shadows. Instead, it has a linear (rather than a non-linear signal gain) at low levels.

I suspect it's on the 2.2 part, because the pushed part really is way left on the histogram. Never a bad idea to ask when one is not sure.

sRGB uses a gamma constant of 2.4 - but with the presence of the low-level offset, comes close to averaging a gamma of 2.2 across the entire transfer-function - whereas Adobe RGB uses a constant of 2.2, and no low-level offset. However, 18% illumination (at sRGB=118) is well above the low-level offset region, and ISO specifications themselves are defined in terms of the sRGB gamma curve itself.

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