Different kinds of RGB

[Want-a-leica]

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In a nutshell, what is the difference between the three options I have on my M8 for Colour Management - SRGB, Adobe RGB and ECI RGB (which I've never heard of)?

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[jaapv]

Small color space - large color space - larger color space. Best to use the largest possible for as long as possible in postprocessing. As soon as you go down from a larger colorspace to a smaller one you lose the colors at the ends of the gamut irretrievably, so you cannot go back. On the camera the settings are only relevant if you shoot Jpeg as the colorspace is assigned on raw conversion.

[adan]

Well, let's clarify a bit. All color spaces are numerically equally large - i.e. one can get a bright red of value 255R/0G/0B in any color space. All color spaces reach to 255 as the max value (or 64516 in 16-bit files)

 

However, the "gamut" of sRGB is smaller than that of Adobe 1998 - i.e. a red of 255R/0G/0B will be a duller, less intense red in sRGB than in Adobe 1998.

 

sRGB's "brightest" red of 255/0/0 is only a red of 219/0/0 in Adobe 1998, so Adobe has room/space/"gamut" of up to 36 units of even brighter reds (220, 221, ...255) that can't "exist" in sRGB.

 

As Jaap says, if you convert an Adobe or ECI image to sRGB, all the reds and other colors that lie outside the sRGB space will be either (depending on conversion technique):

 

"dulled down" proportionally (Perceptual), or

 

compressed so that the tonal distinctions between the brighter (out of gamut) colors all end up the "same" color - ie.e "clipped" to sRGB's 255/0/0 (Relative/Absolute colorimetric).

 

sRGB is an older color space that was tuned to the limited gamuts of printers and monitors dating back to 1996. It guaranteed that all colors were held to a "lowest common denominator" of intensity/saturation that could be reproduced by almost any screen or printer.

 

As devices have gotten better (wider gamuts of inks), the preferred color space has changed, first to Adobe 1998, and then to the even wider-gamut ECI or ProPhoto or the Bruce colorspace or Kodak Ektaspace (tuned to match the color gamut and thus handle all the colors that Ektacolor 100 film can capture).

 

Technically speaking, a color space is defined not by just the color gamut, but also the white point, black point, and gamma (roughly speaking, contrast slope).