Color management for photographers: a primer | Ming Thein

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Starting from the start, a colour space defines all of the possible tones and hues that a single pixel may take. The values may take either a RGB value or CMYK value; the set of three numbers represents the amount of each colour present from a scale of 0-255, which gives 256 possible tonal values for each colour (red, green, blue or cyan, magenta, yellow and black). Combining every possible permutation of these gives you 16.7 million possible different colours for RGB. Note that you don’t get 4.3 billion colours for CMYK, because the K (black) value controls the overall brightness and density rather than contributing another possible hue to the mix. The reason why we have different colour spaces is because they define the limit of reproducible tones for a particular reproduction method – be that screen, web, print, or TV transmission. The most commonly used colour spaces are either a type of sRGB or newsprint CMYK; both of these actually offer very limited reproduction potential. Many of you will notice that most images you see on the web, or in print, are lacking in depth and tonal subtlety; this is simply because the desired tone simply cannot be reproduced, and as a result lands up defaulting to the nearest possible colour value – which is obviously not going to accurately represent the original image. When a colour goes outside the possible range, this is known as gamut clipping. Some of the better proofing software can be configured to display a warning when this occurs; Camera Raw shows a small warning triangle in the upper-right near the histogram. For photographers, the important colour spaces you need to be aware of for digital display are Adobe RGB, sRGB and ProPhoto RGB. For print, it’s whatever variant of CMYK your printer uses. Let’s start with the former. Adobe RGB is the most common wide-gamut colour space available; almost every camera today – even compacts – has the option to output in this colour space by default – use it. sRGB is a limited, mostly web-safe gamut that varies slightly depending on the standard; one camera maker’s sRGB won’t be the same as another, and the sRGB displayed online will be different yet again – possibly depending on the browser, or your system settings, or any one of many other factors. Avoid this wherever possible. The final RGB – ProPhoto – provides the widest of all gamuts; however, most monitors aren’t capable of displaying the majority of the possible colours, and few web browsers support it. I’m going to leave covering CMYK until the section dealing with print. Although the tonal limitations of each colour space depend very much on the specific colour space themselves, it’s safe to say that in general, you’ll see the difference between Adobe RGB and sRGB most prominently in the blues and greens. There’s a particular shade of sky blue that seems to be nearly impossible to reproduce in sRGB, for instance. CMYK has similar restrictions to sRGB, but biases towards cyan instead of green and blue; overall, it lacks the vibrance and saturation that RGB can deliver – unsurprising given that the constituent colours are not red, green and blue! CMYK is used only for print proofing and is never found in camera or monitor; this is because the native components of these devices are formed of RGB photosites or pixels……


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