Demystifying colour (vii) : sRGB vs. Adobe RGB

So we have colour models, colour spaces, gamuts, etc. How do these things relate to digital photography and the acquisition of images? While a 24-bit RGB image can technically can provide up to 16.7 million colours, not all these colours are actually used.

Two of the most commonly used RGB colour spaces are sRGB and Adobe RGB. They are important in digital photography because they are usually the two choices provided in digital cameras. For example in the Ricoh GR III, the “Image Capture Settings” allow the “Color Space” to be changed to either sRGB or Adobe RGB. These choices relate to the JPG files created and not the RAW files (although they may be used in the embedded JPEG thumbnails). All these colour spaces do is set the range of colours available to the camera.

It should be noted that choosing sRGB or Adobe RGB for storing a JPEG makes no difference to the number of colours which can be stored. The difference is in the range of colours that can be represented. So, sRGB represents the same number of colours as Adobe RGB, but the range of colours that it represents is narrower (as seen when the two are compared in a chromaticity diagram). Adobe RGB has a wider range of possible colors, but the difference between individual colours is bigger than in sRGB.

sRGB

Short for “standard” RGB, it was literally described as the “Standard Default Color Space for the Internet“, by its authors. sRGB was developed jointly by HP and Microsoft in 1996 with the goal of creating a precisely specified colour space based on standardized mappings with respect to the CIEXYZ model.

sRGB is the now the most common colour space found in modern electronic devices, e.g. digital cameras, web, etc. sRGB exhibits a relatively small gamut, covering just 33.3% of visible colours – however it includes most colours which can be reproduced by visual devices. EXIF (JPEG) and PNG are based on sRGB colour data, making it the de facto standard for digital cameras, and other imaging devices. Shown on the CIE chromaticity diagram, sRGB shares the same location as Rec.709, the standard colour space for HDTV.

Adobe RGB

The colour space was defined by Adobe Systems in 1998. It is optimized for printing and is the de facto standard in professional colour imaging environments. Adobe RGB covers 38.8% of visible colours, 17% more than sRGB. Adobe RGB extends into richer cyans and greens than sRGB. Converting from Adobe RGB to sRGB results in the loss of highly saturated colour data, and the loss of tonal subtleties. Adobe RGB is typically used in professional photography, and for picture archive applications.

Adobe RGB and sRGB shown in CIELab space

sRGB or Adobe RGB?

For general use, the best option may be sRGB, because it is the standard colour space. It doesn’t have the largest gamut, and may not be ideal for high-quality imaging, but nearly every device is capable of handling an image embedded with the sRGB colour space.

  • sRGB is suitable for non-professional printing.
  • Adobe RGB is suited to professional printing, especially good for saturated colours.
  • A typical computer monitor can display most of the sRGB range but only about 75% of the range found in Adobe RGB.
  • Adobe RGB can be converted to sRGB, but the reverse is not true.
  • An Adobe RGB image displayed on a device with a sRGB profile will appear dull and desaturated.

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