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When comparing the sRGB and CMYK color spaces, the sRGB color space encompasses much more blue than the CMYK does.

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Open Photoshop using sRGB as the color space. Fill the entire image with blue (rgb(0, 0, 255)) and print it out (CMYK). Given that the monitor and printer are accurate, will the printed image be less blue than the monitor?

If this is the case, why is CMYK being used? ProPhoto RGB produces an even bigger range of colors. Why isn't this the standard as it can reproduce more colors to the eye? Why isn't a single color space used where blue is the exact blue (ex. 460nm)?

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@AJHenderson As I see it this is absolutely vital to photography. Photography to me encompasses the whole chain from actually dialing the settings on the camera, to editing the photography on the computer, to sending it to the printer. Is this site only for the camera part of it? –  Friend of Kim Aug 29 at 18:53
    
@AJHenderson: I think a proper understanding of color spaces helps one to not screw up photos during postprocessing and exporting/conversions. Color management and color spaces are important... –  TFuto Aug 29 at 18:53
    
I guess maybe the question needs more context as to what you mean by "why is CMYK being used then". Many high end printers actually do work in RGB for C-types. It is on topic if this relates specifically to photo printing, but it wasn't clear from the context of the question. –  AJ Henderson Aug 29 at 18:55
    
@AJHenderson I picked CMYK and sRGB because it made it very easy to make my point in the question. But this really goes for all color spaces relative to photography. –  Friend of Kim Aug 29 at 18:59
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It's important to recognize that a color space is not 2 dimensional as this plot shows, but 3 dimensional -- the way reproducible color changes as it gets darker and lighter will show that CMYK and RGB are more different than this plot shows. I can't find a good 3D view of the CMYK space, but I hope it's sufficient to say that this is not a complete picture of either color space. –  Dan Wolfgang Aug 29 at 19:58

7 Answers 7

up vote 3 down vote accepted

The covered range of colors (called gamut) is different between color spaces because these ranges are then discretized, "digitized", represented on a fixed amount of bits, and then reproduced on a monitor, in print, etc. One wants to store as much color information as possible in a certain amount of bytes.

Now, if you have an equipment which can only emit in CMYK, e.g. a printer, obviously you do not want to store color values that are way off gamut, as some information is lost. You could store CMYK images in ProPhoto or L*a*b, using 32 bit color representations, on the other hand, you are wasting storage space.

Imagine that you can only display 16 colors with your video card (like old days EGA). How would you choose those 16 colors? Obviously you would check what colors the monitor can display, and then select 16 systematically to cover the largest gamut with that 16 colors. Then you would use four bits to represent each color.

Now, if you know that your monitor can only display that 16 color, you would not store your AdobeRGB, 14-bit RAW images, you would convert those images to the 4-bit-color gamut and store a much smaller file.

Another way to describe this: maybe you work with ProPhoto images but these will be viewed with average TFT monitors. The gamut of the monitor is far less than the ProPhoto gamut. Again, there is no use to transfer and display ProPhoto images on TFT monitors, so you will convert these to smaller images that still cover the entire gamut of TFT monitors. (You will want to work with the largest gamut possible though to avoid artifacts during editing.)

So to sum this up: you want to cover the largest amount of reproduceable colors with the smallest amount of bits, and each color space is a compromise around this.

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So if one has got enough disk space and processing power, the biggest gamut possible is desirable (given the monitor can display all the colors)? If the image is meant for print with sRGB, would it be best to use sRGB because ProPhoto would give you more saturation than you will get on print? –  Friend of Kim Aug 29 at 18:57
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1. Yes, that would be ideal :-). However, the monitors cannot display all the colors, especially violet. (What you see as "violet" on monitors is purple, not violet. Purple is red+blue, violet is a single wavelength color. Many people don't even recognize this. And monitors usually do not display violets well. I have yet to see a monitor that is close to display all the colors in L*a*b. (Which extends the range of visible colors, by the way...) –  TFuto Aug 29 at 19:01
    
So if you take a photograph of a violet object (RAW) and display it with ProPhoto RGB on a good monitor, you'll actually get a different color when you compare the monitor with the violet object? –  Friend of Kim Aug 29 at 19:04
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I understand :-) Thank you for your help! –  Friend of Kim Aug 29 at 19:23
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I finally understand how this is working. Proof setup in Photoshop shows a preview on how it's going to look after a conversion to another color space. It makes sense. –  Friend of Kim Aug 29 at 19:28

CMYK

CMYK is a subtractive colour model rather than an additive as in the case of sRGB. The subtractive colour models are used in printing since they allow dyes, ink or paint pigments to absorb certain wavelengths from an otherwise white surface. The dyes, ink and paint pigments can be a very limited discrete set that are mixed to get a wide range of colours.

enter image description here

CMYK uses cyan, magenta and yellow as well as black (key) as ink in the printing process. A result of using CMYK is that you can't get as saturated colours as in sRGB especially in the blue part of the spectrum.

sRGB

The sRGB is an additive colour model. An additive colour model on the other hand builds upon black and usually three primary colours, red, green and blue are used. Adding all three colours ideally yields white. A result of using sRGB is that you can't get a cyan as saturated as you can with CMYK.

enter image description here

This technique fits computer monitors and television screens since they illuminate an otherwise black screen.

Since these colour spaces are used for different media, they both have a place. Using a single colour model with a single colour space may result in unwanted and unexpected colours if printed or displayed on a media that has to convert the colour model first. This is the case with most printers printing an image defined with sRGB.

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CMYK is based off of offset printing capabilities where you are using only specific shades of Cyan, Magenta, Yellow and Black ink being laid down from separate printing plates to print full color. It isn't mechanically capable of reproducing very saturated colors.

But ... most photo inkjets don't expect you to send generic CMYK files to them. They have their own ink colors, color space transforms and often more than three different color inks (especially photo printers), which allow them to get results outside the traditional CMYK color space.

But ... still there are practical physical limits based on the need to lay down wet ink that may be absorbed by the paper, or fail to dry fast enough if laid down to thickly, or can't be perfectly dithered.

But ... even a "perfect" inkjet would still be limited to using subtractive colors based on the absorption and reflection of light and won't be able to match perfectly what can be represented on a backlit RGB screen -- which uses additive colors displayed through transmitted light.

TL/DR -- It isn't the industry being mean, or making dumb choices, its physics. :-)

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So really if you're doing work for print, you should really use the color space of the printer? If you use ProPhoto RGB for an sRGB printer, you'd get a less saturated image on print? –  Friend of Kim Aug 29 at 19:00
    
Modern color management just asks that all the devices in the workflow to have ICC profiles. That way the computer knows how the monitor displays colors, how the file expects its colors to be displayed and how the printer displays colors. But would a file that uses ProPhoto RGB give different results than a file that uses sRGB when printed? Both files will have the colors mathematically mapped to the device profile, so I wouldn't expect the difference to be huge -- but it is a good question. –  David Rouse Aug 29 at 19:12

Ease of print color mixing. When you are using ink based printing, any application of ink darkens a white page, thus making it difficult to get a true sRGB color space in prints without many inks. If you use a dark blue ink, you can't easily produce light shades of blue. By using lighter shades and a black filler, you can mix the amount of darkness you want (controlling luminance) independently of the color. It prevents a really rich blue as it is really a darkened light blue, but it works better for subtractive color.

Ink based photo printers additionally add different levels of grey ink and different intensities of colors to allow both for richer color inks as well as finer control over the luminance when printing. Printers that are purely CMYK use that combination because it gives an ideal range of colors with a minimum number of inks for print media on a wide variety of paper types.

Displays, on the other hand, are additive, they start from black and add light. They can simply not add any light and have a pure black. (The black is effectively automatically part of the system to begin with.) When they go to full intensity, they combine to produce white since they are producing light rather than simply reflecting it.

It is possible with C-type prints, where photo paper is developed, to actually use an RGB process of emitted light to develop the paper and this is often done on higher end C-type prints, but it doesn't work as well for systems that apply ink to white paper.

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Exact blue is exact blue in all color spaces. If you have a specific color in one color space and convert it to a different color space, it's still exactly the same color (provided that the color is actually possible to represent in the new color space). The color code will be different, but it still represents the same color.

If you create a very bright blue like (0,0,255) in the sRGB color space, it's very likely that it will be outside the color space that your printer can reproduce. If you convert the image to that color space, it will then be converted to the closest color that exists in that color space.

If you create a less saturated blue like (64,64,192), that will likely be reproducable in the printer color space. If you have a calibrated monitor and a calibrated printer, the result will be very similar colors.

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Suppose one wishes to design something with two colors: the most saturated bluish-purple a printer can produce, and another shade of bluish-purple that is 5% less saturated. If one is using CMYK, one may express those colors as (100%/100%/0%/0%) and (95%/95%/0%/0%) and be assured of both getting the most saturated bluish purple the printer can produce, and getting something that is somewhat less saturated. If the document is printed on a device which can render more saturated colors, the colors will be more saturated; if it's printed on a device which can't render such saturated colors, then both colors will be less saturated.

Using RGB instead of CYMK wouldn't achieve the same objective. If a particular printer's most saturated blue would be equivalent to RGB values of (10%/10%/90%) one might be able to use (10%/10%/90%) and (15%/15%/91%) as the two colors one uses with that printer. Printing the document on a device which can render richer colors, however, would fail to take advantage of that capability. Worse, if the document is printed on a device which can't manage anything that's more saturated than (16%/16%/92%), it's possible that both colors might be pegged to that shade or something close to it.

It's possible to avoid "pegging" values at the edge of the printer's gamut, even while using RGB, if one reduces the color intensity of images so that a 90%-saturated color in RGB will map to a color that's 90% as saturated as the printer's most-saturated color. Such an approach can be somewhat workable, but it effectively means that what's called "RGB" isn't really RGB but is instead (100%-cyan-black, 100%-magenta-black, 100%-yellow-black); unless the display software processes the colors as CMYK, the colors are unlikely to come anywhere near close to matching what will end up getting printed.

TL;DR: Using CMYK can specify colors relative to the most saturated ones a printer can produce. Printing on a a machine which can produce more saturated colors will yield more saturated output. Using RGB will generally indicate that printers which vary in the level of saturation they can achieve should attempt to produce the same output regardless. RGB is often better for naturalistic photographs, but CMYK can be better for things that are hand-drawn with saturated colors.

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Firstly, you can't really say "less blue" - monitors and printed ink function under completely different viewing conditions. Monitors are backlit. Printers and their inks also differ wildly in quality. But in some sense, you are right.

CMYK just a mixing model and top quality photo printers don't use it anymore anyways.

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