To ensure the right colours in your images, it helps to understand how colour spaces work… Colour spaces are mathematical models describing the way colours can be represented. The easiest way of visualising them is to think of a box containing all the possible colours that can be produced by mixing the three primary colours of light: red, green and blue.
The International Commission on Illumination (Commission Internationale de l’ø‰clairage or CIE) is the world’s authority on light. Under its aegis, a scheme was developed for modelling colour, based upon human vision. The resulting CIE 1931 XYZ colour space, an early attempt based on measurements of human colour perception, is the basis for almost all other colour spaces today.
This diagram plots all colours visible to the human eye on a two-dimensional base. The colour space within the axes provides a unique position for every colour that can be created by combining red, green and blue. (Source: Paulschou at en.wikipedia)
Colour spaces are normally shown graphically as RGB diagrams in which the amount of green is mapped along the one axis, the amount of red on another and the amount of blue on a third, with saturation increasing outwards from the centre. To fit three axes to a two-dimensional format, mathematicians have come up with a diagram that looks like the illustration below.
This diagram shows the different gamuts of the colour spaces most commonly used in photography. Note how the ProPhoto RGB extends beyond the limits of human colour vision in blue and green.
It’s important to understand that colour spaces simply define the colours that can make up image pixels. They do NOT store colour and they are often smaller than the ‘gamut’ (range) digital cameras can take in.
You need to set a colour space on your camera and also for any output device (monitor, printer, etc) so they all attribute the same colours to the same pixels. If different colour spaces are used for capture, editing and output, colours won’t be reproduced accurately.
Interestingly, raw files don’t have a colour space; you can set whatever colour space you want. And they can usually record the full gamut of colours in the scene, which will be wider than most colour spaces.
Who Uses Colour Spaces?
Different creative pursuits use colour differently and, therefore, utilise different colour spaces. Photographers are most concerned with transmitted light (made up from red, green and blue) because it is used to create images ““ and also view images on screens. The colour spaces most commonly used in photography are sRGB, Adobe RGB and ProPhoto RGB.
But when images are printed, the focus shifts to reflected light and the colours used are cyan, magenta and yellow, which are ‘subtractive’ colours. Cyan results when all red is removed, magenta comes from the removal of green, and yellow is the antithesis of blue. An additional ‘key’ component, black (denoted by K) is required to compensate for the fact that no pigments, inks and dyes are pure, so you can’t obtain a true black by simply mixing cyan, magenta and yellow.
The resulting CMYK colour space is mainly used by the printing industry, where it is all-pervasive. It’s found in all printers from the simplest inkjet to the most complex commercial printer.
Video producers require a more complex model to handle colour, with xvYCC (or Extended-gamut YCC) being the international digital video colour space. In this colour space Y indicates the luminance (brightness) component while the two C components are the blue-difference and red-difference colour components.
HSB (Hue, Saturation, Brightness) is often used by artists because they find it more natural to think about a colour in these terms than using additive (RGB) or subtractive (CMYK) colour components. HSL (Hue, Saturation, Lightness/Luminance) is a variant.
‘Working’ Colour Spaces
The most commonly used working colour spaces for photography are sRGB and Adobe RGB and more sophisticated cameras will offer both. But which should you choose?
You only need to make this choice for JPEG files (or TIFFs) as you can select whichever colour space you want when converting raw files into editable formats. So what are the differences between them?
sRGB was created in 1996 by HP and Microsoft for use on monitors, printers and the Internet. It’s the default colour space in all digital cameras and scanners as well as photo printing kiosk monitors, and may be the only colour space supported by many of these devices.
The sRGB gamut (range of colours that can be reproduced) meets or exceeds the gamut of almost all inkjet printers and computer monitors as well as most TV screens. This makes it the best colour space to use for images that will be viewed online or on computer monitors or TV sets, or printed outside the home.
Adobe RGB was developed by Adobe Systems in 1998 to encompass most of the colours achievable on professional colour printers. It covers a slightly wider colour gamut than sRGB, mainly in the cyan-greens. This makes it the preferred colour space for landscape photographers.
Unfortunately, if the monitor can’t display the additional colours encompassed by the Adobe RGB colour space it will force these colours back into the sRGB colour space. The result may not be all that pleasant.
However, images edited in Adobe RGB should be able to produce greater vibrancy in blues and greens than the same images printed in sRGB, because you can capitalise on the wider gamut. Raw files should be converted into 16-bit TIFF files, rather than 8-bit JPEGs because higher bit depth makes more colours available.
Colour Spaces for Editing
When deciding on a colour space for editing images, wider gamut colour spaces have the advantage, particularly when you’re working with 16-bit TIFF files. The sRGB colour space can encompass roughly 35% of the full range of reproducible colours defined by the CIE, which is more than enough for web use.
However, any colours outside the sRGB gamut will be clipped or compressed and, therefore, effectively eliminated. Adobe RGB can accommodate about 50%, which is more than many monitors can display. There will be much less clipping with this colour space, although some subtle hues and tones at its periphery may be lost.
Using a larger colour space enables users to retain the full range of colours their cameras can capture in raw file format. This will help prevent the posterisation and data errors that can happen when a wide colour gamut is crammed into a restricted colour space like sRGB.
But you must account for the reduction in colour gamut that happens when images are printed because prints by their nature have a restricted colour and tonal gamut, which is determined by the particular printer/ink/paper combination. In most cases colours can be mapped to what the printer can manage, through choice of ICC profiles and rendering intents.
There’s been plenty of debate about whether wider colour spaces provide any ‘real world’ advantages. The consensus is that if you do your own printing, Adobe RGB will provide as much colour gamut as you can see on your monitor and probably more than can be output in a print. However, you may be able to obtain better reproductions of images with very fine colour and tonal nuances if you opt for a wider colour space, particularly with B&W conversions.
Workflow Management
Colour spaces must be the same throughout a photographer’s workflow to ensure consistent colour rendition. If you capture, view, edit and print images in the sRGB colour space, you can usually ignore most colour management controls and still end up with nice-looking images. If you capture in Adobe RGB and view your images on an sRGB monitor, the colours may not look as good.
Colour management is designed to integrate colour reproduction throughout the digital workflow. It becomes very important if you work in colour spaces with wider gamuts, particularly for images containing saturated colours where you will need total workflow control.
Sophisticated image editors can handle very wide colour spaces, with ProPhoto RGB being the most popular among photographers as it is one of the options available in Adobe Camera Raw. It is, therefore, supported in Photoshop.
The Camera Raw converter in Adobe’s Photoshop offers four colour spaces, shown in the above screen grab.
Wider gamut colour spaces like ProPhoto RGB can include a percentage of colours that don’t exist and can’t be reproduced. If they are not brought back into a gamut the output device (printer or screen) can handle, many images will show posterisation effects.
Although some of the latest monitors can display most of the colours in the Adobe RGB gamut, the distribution of colours and tones is not necessarily even, which can produce images that don’t look like those that are printed. Most current desktop printers with expanded ink sets can print colours beyond the sRGB gamut ““ but they can seldom match the gamut and dynamic range of camera images.
The Advantages of High Bit Depth
Regardless of which colour space you use, the range of hues and tones that can be represented will be influenced by the bit depth in the image file. JPEGs are limited to 8 bits/channel and, with 256 possible values in each channel, each file has the potential to record a total of 16.7 million hues and tones in discrete pixels.
Raw files can be 12- or 14-bit but they are usually converted into 16-bit TIFF files which can encompass a massive 281 trillion possible colours. However, a 16-bit TIFF file doesn’t provide additional colours to fill the expanded gamut, it simply packs a lot more discrete hues and tones in between each of the established 8-bit colours.
This article is an excerpt from Photo Review magazine Issue 59
