Is Nikon Mode III really sRGB?

Started Apr 6, 2005 | Discussions thread
OP Bill Janes Senior Member • Posts: 1,848
Re: Is Nikon Mode III really sRGB?

John Yu wrote:

Nikon has made it confusing. The colour modes don't only change the
colorspace, but also how the in-camera image processor interpret
the colours in the raw data.

Both mode I and mode III produces the data in the same sRGB
colorspace. But the output colours are generated differently. for
example, a particular shade of red is rendered as sRGB(240, 2, 2)
in mode I, it is rendered as sRGB(231, 1, 1) in mode III. (I made
up the values in this example. They're not real.)


I think that your explanation is correct, but Nikon appears to be using non-standard color spaces. If we are shooting in raw the actual data in the stored file is the same regardless of the mode, since the files are merely tagged with the selected mode.

As I understand things, the generation of the output values of RGB is done according to the selected color space. The actual math can be a little convoluted, but the definition of the space is pretty simple. It involves specifying the gamma, white point, and primaries in CIExyY as shown in this screen capture from PhotoshopCS.

Interestingly, the olny difference between sRGB and aRGB is in the green primary, where aRGB has extended range. Since mode III shows green to advantage, it must be using a diferent green primary than is used in Mode I, more like aRGB.

These details are explained in great detail on Bruce Lindbloom's site, and he shows how to convert between spaces and has an online calculator to do the math.

My assumption that Adobe's sRGB was standard was incorrect. They use a simplified sRGB for gamma correction. In the actual sRGB specification a linear equation is used for low values, and a power function with a gamma of 2.5 is used for higher values to give an average gamma of 2.2. Adobe sRGB and aRGB use a gamma of 2.2 for all values. I don't know what Nikon does, but it appears to be nonstandard.
Bill Janes

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