Friday, 18 July 2014

Choosing the right Colour Space

When it comes to the choice of a colour space for the digital photographer, you’ll find a great deal of confusion.  This is understandable because it’s a complex topic.  Actually finding the truth about colour spaces gets tougher simply because there’s an awful lot of mis-information about colour calibration circulating the ‘net.  So, it’s not surprising that many have no idea what a colour space is, or whether it’s of any use to us.

Definition: On a superficial level, a colour space describes a range of colours in a particular colour model. This model might be the RGB seen on all computer displays (i.e. sRGB) or it might be the model used to describe an offset litho press (CMYK).

Here's a standard colour model illustrating the different colours available through different colour spaces.
It also includes an indication of how matt printing paper reproduces colour - all four spaces have unique colour ranges.
To help understand this concept, a colour space can be displayed in three dimensions by showing cyan along the X axis, magenta along the Y axis and yellow along the Z axis (for example, from the CMYK print colour space).  This provides a 3D model demonstrating the range of tones possible through a combination of all three colour axes.

Though a somewhat childish analogy, printing is like using a palette of 24 crayons. But if you start out with 36 or 48 crayons, which ones go and which ones stay when you downsize from a wide gamut space to a narrower one...
On a purely simplistic level, a colour space is a bit like having one giant colour crayon set containing 75 crayons.  Let’s say this represents a wide gamut colour space like Pro Photo RGB.  To represent Adobe RGB, which has a narrower gamut (range), you might remove 25 crayons.  The smallest colour space used by photographers is sRGB so this might only have 35 crayons. (Note: The precise numbers here are used only as an illustrative tool).

The missing crayons in the Adobe RGB and sRGB analogy are not just from one colour region but, in the example comparison between sRGB and Adobe RGB, they come from the green/cyan part of the crayon box.  So looking at the 2D illustrations of what colours these different spaces represent, you can see exactly how much more green/blue Adobe RGB has when compared to sRGB, and how much more of the same colours ProPhoto RGB has over the narrower gamut Adobe RGB.

But before we get too excited by this newfound knowledge and rush to change everything to ProPhoto RGB, here are some basic colour space facts:
- ProPhoto RGB certainly does represent a very wide range of possible colours.
- sRGB however contains sufficient colour information for most practical applications.
- Most digital cameras only offer sRGB and Adobe RGB colour spaces in their feature set.
- Most consumer monitors can only display the sRGB colour space.
- Therefore you cannot physically see, let alone appreciate, the additional tones found in a broader colour space on a monitor.
- Most consumer printers can only reproduce colours from the sRGB colour space
- RAW files are definitely the best file format to use.  They save approximately five times the amount of image data as a JPEG.
- If the file is being used for the web it has to be converted to the sRGB colour space anyway, because that’s the default colour space for the web.
- JPEGs, because of their shallow bit depth, are always sRGB
If you digest the above facts, it would seem that most of our photographic lives work best in the sRGB colour space.  There seems little logical reason to set cameras to a colour space that produces a range of tones that cannot be displayed, nor seen correctly under normal viewing conditions.  It then follows that choosing a space that is wider than sRGB could be a waste of effort.
My thoughts on this are that it’s probably better to choose the sRGB colour space throughout your workflow.  Doing this ensures that at no time do you have to compromise what you can already capture, see onscreen or print.

Others espouse the benefits of shooting Adobe RGB and editing in that same space under the impression that they will retain the best possible outcomes.  I think this is a pipedream because at some stage in the edit process, you will have to compromise the colour space in order to output/upload to the web, to print (inkjet), or to publish (offset). 

So clearly when the colour space is reduced from Adobe RGB to sRGB, we need to mitigate any colour and contrast loss using very careful editing techniques.  To do this we have to investigate individual colour channels, something that you can do with Photoshop but not using Elements.

sRGB is the easiest and perhaps most reliable colour space in which to work because being the smallest, it presents few colour limitations (and fewer nasty surprises) when outputting to the web or any kind of consumer print.

Adobe RGB is a wider space - but care must be used both at the editing stage (to preserve colours when downsizing to sRGB for print) and especially so once the file is being handed off to a third party simply because there's guarantee that the person receiving the file knows what they are doing either.

ProPhoto RGB can encompass a huge range of colours but great skill is required to process these files in order to retain otherwise fragile tones once the file is output to the web or print. You can retain much of this space’s colour gamut in print but this also requires both high-end printers and skill in executing the process.

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