sRGB and AdobeRGB (and ProPhoto RGB) are ‘colour spaces’. What’s that all about, and why am I always told to set my camera to AdobeRGB if I want to print?

It’s all about the trade-offs we make when we try bring together colours we humans can perceive and a numerical representation of colours in a digital system.

We humans can distinguish colours because the sensitive parts of our eyes are made up of a huge number of light-sensitive organs called ‘rods’ and ‘cones’. Rods sense intensity of light. But there are three kinds of cones: one kind is sensitive to long wavelength light (red colours); another kind os sensitive to medium wavelength colours (greens); and the last kind is sensitive to short wavelength colours (blues). We can make out millions of different colours because our brains interpret the combined effect of light on the rods and different kinds of cones.

Essentially, for a digital device like a computer, monitor, printer, fax etc etc to deal with colour (or any concept) the colour has to be able to be presented uniquely numerically – because computers work with numbers.

Mimicking the eye, we can get a computer to record a colour by assigning 3 numbers to it. A convenient set of three numbers is a set that represents ‘the amount of red’, the ‘amount of green’, and ‘the amount of blue’ that goes into making up the colour. Thinking about colour that way is to use a so-called RGB colour model.

Now, the numbers you can use are defined by the sensor or the computer hardware or the software. In the early days computers were 8-bit devices. The numbers you could use to represent ‘the amount of’, ran from 0 to 255. A colour would be represented by a triple of numbers — like (135, 64, 200) — representing the ‘amounts’ of red, green, and blue in the colour. That means you can uniquely describe 256 x 256 x 256 (= 16,777,216 ) different colours. (If you really want to know why 0—255 in an 8-bit device, then here it is!)

However, the colour sets (spaces) — i.e. which particular colours — to which you apply these numbers, are a matter of choice! A useful analogy is to think of a boxed set of coloured pencils. Suppose our boxes can hold only 100 pencils. You could put one set of colours in one box, and a different set of colours in the other box. Same number of pencils, but a different colour set.

The range of human vision (so called ‘visible colours’) is to all intents and purposes infinite. Too large to have a unique number assigned to every real colour.

So someone defines a unique colour to each of the 16,777,216 number triples that are ‘in the box’ or set. Those colours are a colour space.

sRGB was one of the first definitions off the block. Microsoft and a few others looked at the range of colours that the then available monitors could display and decided to allocate all 16,777,216 number triples to colours within that range. The range, in a sense, is defined by the ‘most red’ and ‘most green’ and ‘most blue’ colours that are described and which are represented by (255, 0 0) , (0,255,0) and (0,0,255) respectively.

Adobe and Kodak and others recognised that printing machinery is able to display more saturated colours than those contained in the sRGB space. So AdobeRGB was defined, which would assign the 16,777,216 triples over a different set of colours containing more saturated colours that could all be printed (even if not all of them could be seen on a monitor).

Kodak went even further with ProPhotoRGB which assigns numbers to reds, blues, and greens, that are more saturated than even printers can reproduce yet. But they are colours that the camera can catch and that we can see. ProPhoto’s uses are only ‘under the hood’.

Nowadays computers are more powerful so that many more than 16 million unique colours can be defined because colour is represented in 12-bit arithmetic. 68 billion plus unique colours can, now, be dealt with.

But the range of colours — which colours to put into the box — are still constrained by the choice of colour space. Even in a 16-bit system an image represented in the sRGB colour space can’t contain all the colours that can be printed on an inkjet printer.

The following set of pictures show the relative amounts of colours in the common colour spaces compared to the colours humans can see, and compared to the colours that can be printed on a good semi-gloss photo paper by an Epson 3880 photo printer.
                                            

The pictures above show, in turn:

• the ‘gamut’ of human vision: the colours we can see;
• the gamut of the sRGB colour space compared to human vision;
• the gamut of the Adobe RGB colour space compared to human vision.

These two pictures show the colour gamut of the Epson 3880 printer (the horse shoe shaped space) by comparison to both the Adobe RGB colour space (left picture) and the sRGB colour space (right picture).

You can see that the printer can print many more colours than are contained in sRGB’s “pencil box”.

A camera’s sensor can record a range of colours that is essentially as wide as human vision. Whenever we use a computer (or our camera) to convert colour data from a large colour space to a smaller colour space, we throw away the colour information that is outside the new, smaller, space.

The so-called out-of-gamut colours are changed (rendered) to the closest ones within the gamut of the new space. It is not possible to convert the data back again, because no record is kept of the colours that were abandoned.

If you ever intend to print one of your photographs, never convert the data to sRGB unless you are informed that the printer cannot print any wider colours*. Always keep your image in the biggest possible colour space. When you print an sRGB-encoded photo, colours that were in the scene but which are beyond sRGB’s range, will have been ‘clipped’. Those colours will be ‘wrong’.

If you are shooting and recording in JPEG with your camera, set it to record in Adobe RGB. Make sure that, if you are editing that file in Photoshop, your working space is set to Adobe RGB (or bigger). Abandon sRGB!

If you shoot and record RAW images, then set your working space to ProPhoto RGB which contains an even greater range of colours than Adobe RGB. That method will give you the greatest likelihood of being able to print accurately all the colours that the printer can actually deal with.

The picture shows the gamut of the ProPhoto colour space in comparison to human vision. ProPhoto has a very much wider gamut than AdobeRGB, sRGB or the printer gamut. It will retain the most accurate record of the colours you photographed.

* If that’s the case, you might be better to find someone who has a better printer!