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When I started to manage my photos with lightroom I also got a spyder5 to calibrate my monitors. I got into discussion with a graphics designer and from this I got a question.

Calibrating the monitor was for sRGB according to my software (it showed me the result on an sRGB graphic). I was thrilled because it was so easy and recommended it to the designer. He told me he cannot use it because he needs calibration for CMYK. He also told me CMYK is a 100% subset of sRGB.

So if it is a 100% subset and if Photoshop is able to convert (or work) in CMYK then is it necessary to calibrate the monitor to CMYK as well? As a subset then shouldn't it be able to do a conversion and show it on a sRGB-calibrated monitor correctly?

As I only use it for photos I understand that I only need sRGB. I would just like to know if it is true that calibrating to CMYK is different than calibrating sRGB and letting photoshop do the conversion and still show correct colors on the monitor?

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RGB and CMYK are very different color models. For instance, RGB is additive: you basically start with black and add colors. You combine all colors to make white. But CMYK is subtractive: you start with white and add colors to generate black. If you think about it, it makes sense: Monitors are black by default, when there is no light, the screen is black. Paper is generally white by default, and colors are added to it with printing.

Understanding this, it is then easy to understand that monitors are RGB, and printers are CMYK. You can't change a monitor to CMYK, since it doesn't work that way, literally.

Calibration simply sets your monitor to a neutral state, so that it matches a reference. This allows you have your monitor in a known state, and ensures consistency.

Photoshop shows things on screen in RGB (usually Adobe RGB) for obvious reasons, but when you are ready to send to pre-press, you convert to CMYK, which is what the vendor's printer is expecting. This conversion needs a 'translation' that tells it how to match the RGB colors to CMYK colors. Photoshop has a table, but it needs to know how that table compares to the printer.This is called a color profile or ICC profile.

The ICC profile allows Photoshop to interpret colors on the screen to colors on the printer, and also you to interpret what you see on screen to be close to or indicative of what is printed. The thing is, these profiles are very specific, down to the printer model, ink used and paper used. For home printers, there are generic profiles that are usually better than nothing, but for pre-press, you need to get sample files and prints, to compare on your monitor so you can dial in your calibration to match the output of the target printer as closely as you can. Note that this is needed for each vendor/printer, and some even do it before each 'run', especially if a run is going to be extensive ($$).

So, your designer is partly right: he can not rely just on the calibration of your monitor, since that is calibrated to a known standard for monitors. You need to go the next step and get the ICC profile and then compare the output to what the printer outputs. Many calibration hardware can do this as well, known as printer calibration, where an images is printed, then scanned by the colorimeter, and an ICC profile is created.

No, you don't calibrate your monitor to CMYK vs RGB. You first need to calibrate you monitor using a hardware calibrator (not the squint method) , then get a ICC profile from the printer, and finally a sample file and print combination from the print shop to compare.

  • So spyder5 is not a hardware calibrator? (btw what does "squint" mean? - is it what spyder does - not precise enough?) and the results of spyder cannot be used as calibration for a print-workflow monitor? – Uwe Hafner Dec 1 '15 at 14:28
  • yes spyder 5 is a hardware calibrator. 'Squint' is the calibration that is available on the Mac and Photoshop that does a calibration where you compare and adjust your monitor based on color patches shown. For several you need to 'squint' to see the changes. ex: computer-darkroom.com/colorsync-display/colorsync_1.htm – cmason Dec 1 '15 at 15:44
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    Perhaps nit-picking, but I think the clarify is important here because the question blurs them: RGB and CMYK are color models, not color spaces. – Dan Wolfgang Dec 1 '15 at 16:09
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This is a very incomplete answer, but important in regards to terminology and understanding everything: RGB and CMYK are color models. They don't define what your monitor or printer can do, only how color is created. A great question to review: What is the difference or relation between a Color Model and a Color Space?

He also told me CMYK is a 100% subset of sRGB.

CMYK is the color model. sRGB is a color space. These are incompatible ideas.

But, for a moment, lets suppose that a CMYK color space is being used, such as a RIP to an inkjet or an offset color printer. Can this CMYK printer profile be contained entirely inside sRGB? That is, is the gamut of the CMYK device smaller than sRGB?

That needs to be determined for each device in use. In general, the CMYK color model (and output devices) are typically thought of as much more restrictive than what an RGB device is capable of. But this isn't entirely true or a complete picture.

Many monitors display the gamut of the sRGB color space adequately. Some display the Adobe RGB space completely. Few delve further, into the realm of what can be contained in ProPhoto RGB. Output devices are equally varied: a 4-color inkjet printer is going to have a relatively small small gamut, where a 7-color inkjet will have a notably larger gamut (cyan, magenta, yellow, and black inks vs cyan, light cyan, magenta, light magenta, yellow, black, and light black inks). Similarly, prints can be made with 8 or even 16 colors. In fact, inks can even be specially mixed -- printing with a variety of blues and purples can also overcome the relatively small number of blues that a traditional CMYK device can output.

So, it is entirely possible that your designer friend's CMYK output device/methods fall within the realm of what the sRGB color space can show, but that's hardly a true or representative statement.

  • Good clarification. From all the answers I start to understand these theories and I have to admit that maybe the "s" in sRGB was interpreted by me. Possibly the designer was talking about RGB. I will show him all the answers and with all the information we should be able to sort this out. – Uwe Hafner Dec 2 '15 at 8:32
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In my opinion the designer is wrong.

CMYK is NOT a subset of sRGB. It is a diferent color model.

Let me explain.

sRGB is a color profile for rgb colors. But actually it is close to a "color space" (yes, this afirmation will be controversial).

On top of that you "profile" your monitor and computer conected together.

But there are tons of diferent standards for CMYK aswell. You first need to define what CMYK you want to simulate on an rgb screen. This is not defined by your operating system (like the monitor) but by the aplication you are using (for example Photoshop) If you change the CMYK profile in your aplication, then your file will be simulated acordingly. (The key word here is simulation).

After that, you can convert your RGB file to CMYK using thoose profiles.

If you use a diferent CMYK profile your values on the converted CMYK channels will be diferent.

If you calibrate well your RGB monitor, the aplication (Photoshop) will simply use its internal system to display CMYK simulation as colse as possible.

Then, a separate step is to profile your printer using again a hardware to do it. Actually you need to profile a printer+inks+paper combination.

At the end you have 4 profiles.

1) The sRGB one.

2) Your specific Monitor.

3) A CMYK generic profile (like Fogra, Swop, Gracol, Eurocoated)

4) A specific profile for your printer.

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