Maximize your colour gamut
Hans Weichselbaum looks at how to choose the right profile to get the most out of your gear
Colour management was a huge topic a decade or two ago. You had to be quite an expert if you wanted to get a decent colour output from your monitor and printer. Today we are used to buying our hardware off the shelf, plugging it together and expecting perfect results. In most cases the outcome will look decent, but are you sure that you get the best possible outcome?
Monitors and printers have improved tremendously, not to mention the quality of high-end DSLRs. But wiring everything up without worrying about the settings will leave you with mediocre colour quality because the system will default to a rather small common denominator.
In this article I am not going into colour profiles and how to match them up (I assume that my professional readers got that right), but will instead focus on the working colour space. And that’s where colour gamut comes in. Colour gamut refers to the range of colours that a device can produce. In most cases this range will be smaller than the colour spectrum the human eye can handle. But technology has advanced, and you are throwing a lot of colours away if you stick with any default settings.
The diagram in image one highlights the importance of the working colour space which sits right in the middle between camera, monitor and printer.
Needless to say, if your import quality is poor, or if you work with a 10-year-old printer, you won’t need to worry too much about the colour gamut of your working space.
There were more than a dozen of those working spaces in the early days of digital imaging, but today there are basically three left for the photographer to choose from: sRGB, Adobe RGB (1998) and ProPhoto RGB.
Image two depicts a simplified diagram showing you ProPhoto RGB has a larger gamut than Adobe RGB, which in turn can handle more colours than sRGB. The diagram also shows you the rough outline of colours you can expect on matte paper from your inkjet printer. Glossy and lustre paper have a wider colour gamut, newspaper print has a much smaller range.
Looking at this diagram you would think that we should all go for the super-large ProPhoto working space, it certainly covers all the colour shades your camera can produce. It even handles colours that not even the eye can see. In practice it’s not that simple, and there are certain pitfalls which can totally mess up your colours.
Let’s first look at this horseshoe-shaped colour space, which represents the visible spectrum in image three. This is called a CIE xy 1931 chromaticity diagram, and the curved edges represent the spectral colours, going from 380nm (violet) to over 700nm (red). The full colour gamut also depends on brightness, and therefore needs a 3D representation, as in image four.
This is a projection of the LAB colour space (a very weird colour mode, different to RGB, which only makes sense to computers). The ‘a’ axis runs from magenta to green and the ‘b’ axis covers the yellow-blue range. The perpendicular ‘l’ axis goes from pure white on top to pitch black at the bottom.
But what stands out in this diagram is the prominent increase in colour gamut going from sRGB (the solid colours) to Adobe RGB (wire mesh). The difference is especially pronounced
in the blue-green region in medium and lighter colours, but also in the darker red-orange tones.
What happens if your camera can produce colour shades which are outside the sRGB range, if that is your colour working space? The answer is you won’t notice much, if anything at all — the colours will look saturated, but you might have lost subtle image details which would be visible if you had chosen a larger working space. In technical terms these colours are “out of gamut” and they get “clipped”. It’s not the end of the world and your colours are still bright and saturated, but you’ve thrown away the extra money you spent on your fancy DSLR and printer.
What colour space are you working in?
Do you know? I’ve been speaking with a number of photographers, publishers, and printers, most of them highly skilled in Photoshop, and many could not tell me what colour space they’re using. Some didn’t even know what I was talking about.
If you haven’t changed your colour set-up in Photoshop then you are working in sRGB. This is Adobe’s default setting. The ‘s’ stands for standard, but it could also stand for small, because sRGB is a rather small colour space with a narrow gamut.
You might have guessed that I am a great advocate of a larger working space than sRGB. Most digital image experts recommend using the significantly larger Adobe RGB (1998) colour space, and this has grown into something like the standard for professionals, printers as well as photographers. It simply means that you get a wider range of colours, in particular more saturated shades of blue and green, but also more intense magenta, orange and yellow tones. Today’s inkjet printers can print most of those extreme colours, and it does make sense to switch to a larger colour space.
How do we select the colour working space? Go to Colour Settings under the Edit menu (or under the Photoshop menu for Mac users) and you will get the interface in Image 5.
Look for Adobe RGB (1998) in the dropdown menu for the RGB working space. Under Colour Management Policies I recommend to switch to Convert to Working RGB. This will convert all imported images into your working RGB if they are not already in the right colour space, no matter what profile they have attached. It is also a good idea to tick ‘Ask when opening’ if there is a profile mismatch. This ensures that you always know what Photoshop is doing in the background.
In addition you need to make sure that the images coming from your camera and your scanner are in Adobe RGB. Look at your scanner settings. Under colour management you should find an option where you can select the colour space.
All professional digital cameras, and even some point-and-shoot cameras, will give you a choice between sRGB and Adobe RGB. The default is always the sRGB setting, and you need to change this in order to get the best out of your camera. This is not strictly true if you shoot in RAW, because the colour space setting in the camera is irrelevant. The colour space is chosen during the RAW conversion (we are going to look at this later). But if you shoot in JPEG, the colour space setting is critical, and you will not get the full potential of your camera if you leave it at the default sRGB setting.
When do we use sRGB?
If sRGB is so much smaller, why bother using it? It must have something positive, otherwise it would not be the default setting of all editing software and digital cameras. The reason is that sRGB is the standard colour space used in all web browsers and any of the many non-colour managed programs, such as word processors and most publishing software. It is also the default colour space in conventional photo-lab printing.
And here lies the danger: if those applications get an image tagged with Adobe RGB, the colours are going to come out dull. So, we get quite the opposite of what we expect. If you work in Adobe RGB, your workflow must be strictly controlled. All photos you email which are going to be viewed with a normal image browser, or images that are going onto a website, need to be converted to sRGB. The same goes if you use an external printing service (which does not expressly state Adobe RGB).
In the end, it does make sense when even Adobe Photoshop chooses the smaller sRGB colour space as default. If you do your own printing or hand your images to a professional publisher/printer, the larger Adobe RGB is the way to go. If you have one of the latest inkjet printers, you are likely to notice the enhanced gamut. On the other hand, the casual amateur is better served with sRGB, and there is less danger of any colour mismatch.
If you need to change the profile of an image from one colour space into another, you do that in Convert to Profile under the Edit menu (don’t use the Assign profile command) as seen in image six. Of course, you can run this as an action if you have a bunch of images to convert.
As already mentioned, RAW files don’t have any colour space, but they do get one assigned during the RAW conversion.
Image seven shows you how to select the colour space in the Adobe Camera RAW (ACR) converter. You will find a similar option in Lightroom. You get four choices: the already familiar sRGB, Adobe RGB, and ProPhoto RGB, as well as the very seldom-used Color Match RGB.
What about ProPhoto RGB?
This colour space is simply huge, much bigger than Adobe RGB. So why not use it? If you work in 16-bit mode, each of the three channels can handle 65,536 shades of that colour (216). This is more than enough to accommodate even the huge number of individual colours that ProPhoto RGB offers. However, the moment you switch to eight-bit you only have 256 levels per channel, and if you have some of those extreme colours in your image, you can get banding (posterization) in areas where you would expect a smooth gradient.
Although ProPhoto RGB will pull even more colours out of your camera (at least in theory), workflow considerations also become more critical than in Adobe RGB, and it is recommended that you keep everything in 16-bit. The advantages in terms of squeezing more colours out of your gear are very small compared to the more complex workflow and the danger of running into banding problems. For these reasons I don’t recommend the ProPhoto RGB working space, unless you have a well-controlled workflow for images going onto the internet, to clients, and external printers.
Your choice of working colour space is critical if you want to utilize your camera gear to the fullest. Also keep in mind that most magazines and book publishers will accept files tagged with Adobe RGB. Take special care when shooting in JPEG — if your camera is set to the default sRGB working space, you are throwing colours away which you cannot recover at a later stage.