Colour range limitations
Another challenge arises from the different limitations of different technical devices in capturing and reproducing the fantastic colour and tonality of light in the natural world.
No real physical device can display all the colours of nature. Cameras cannot detect, and neither can monitors or printers reproduce, the complete range of colours and tones that the miraculous human eye can detect.
Because of the completely different technologies they use, the range of colours a camera can record, the range of colours that a monitor can display, and the range of colours a printer can print, are all different! Each has a different, so-called, colour ‘gamut'.
Colour gamut differences mean that there are some colours that cameras detect that cannot be reproduced by printer or monitor, and some colours that printers can print that can’t be seen on all monitors, and vice versa!
Given the current state of printer and monitor technology, it is simply not possible to get identically the same colour range on a monitor as on a printer.
The pictures show the gamuts of a typical reasonable quality computer display and a good photo printer.
As you can see on the pictures there are many colours that the printer can display that the monitor cannot.
The colours that a device cannot reproduce are called out-of-gamut colours.
The good news is that most of the colours which occur in the real world, and which feature in most photographs, are within the gamuts of both good monitors and good printers.
Typically the only colours that devices have difficulty reproducing are highly saturated colours, as the pictures show.
A well colour-managed digital system should be able to match all of the colours that are within the gamuts of both devices.
A good system will also be able to adjust ('render') any out-of-gamut colours in ways that ensure that any image produced by the monitor, and the same image reflected from a print, both convey the same ‘realistic’ sense of the real-life scene that you photographed. They should look ‘the same’ in the sense that the features of the photo we think important, such as shadow detail, colour balance, and tonal range, create the same impression on a screen as in print.
This is what colour-management is about.
When you send an image to a printer, how any out-of-gamut colours are dealt with, depends on the rendering intent that you choose.
Most photo printing applications allow you to choose at least two rendering intents: relative colorimetric, and perceptual. There are two other possible rendering intents: saturation, and absolute colorimetric but they are seldom used in photography.
The perceptual rendering intent compresses all colours so that the most saturated out-of-gamut colours in the image are rendered as the most saturated colours (of the same hue) the printer can produce. All other colours are scaled back proportionately. Some printed colours will, deliberately, not be the same as those in the image, but the overall tonality and colour relationships in the image are maintained.
Relative colorimetric rendering intent essentially assigns all out-of-gamut colours to the nearest in-gamut colour of the same hue, and leaves all other colours unchanged. This intent preserves colour accuracy but can destroy colour balance if an image contains a lot of out-of-gamut colours.
If your printing software has a 'soft proofing' capability (where the printer gamut is simulated on your monitor) you can choose the rendering intent that best suits the colour distribution in your photograph.
There's a good discussion of rendering intents, and soft proofing, on site.