20 Replies Latest reply on Sep 9, 2014 9:34 AM by sPretzel

# Colour shift (from CMYK = 0 0 0 20 to Grayscale K = 22 !)

Hello,

On an empty canvas, in CMYK mode, I create a new fill layer. I fill this layer with colour CMYK = 0 0 0 20. I then check the colour with the eyedropper tool. It reads 0 0 0 20.

I then go to Image>Mode<Grayscale and convert the image from CMYK to Grayscale mode. I check the colour with the eyedropper tool. It reads K = 22.

Why is there such a shift? I am expecting K = 20 in Grayscale mode. Thanks.

This is for CS4.

• ###### 1. Re: Colour shift (from CMYK = 0 0 0 20 to Grayscale K = 22 !)

Where do you see the K = 22? I got CS6 and when I tested what you did, I saw this:

So far, so good. The left info field is set to grayscale. Means 20% K in CMYK is equivalent to 21% K in grayscale. However, after the conversion the K value of grayscale does not change, but the CMYK values do.

If you now pick the colour, change it again to CMYK 0-0-0-20 and fill, the info does not change, because it is not in CMYK mode. You can't directly change the K level in grayscale, as it seems

When using CMYK 0-0-0-100, the K level in greyscale will be only 95. If you would alternatively fill with RGB  0-0-0, then K = 100. Strange!

PS actually sticks to the rule CMYK = 0-0-0-100 is the same as RGB = 0-0-0 and CMYK = 0-0-0-0 is the same as RGB = 255-255-255, but with 20% black as CMYK = 0-0-0-20 it suddenly corresponds to RGB = 217-217-217, but it should be 255-(255/5) = 204, means RGB = 204-204-204. Hmm, this makes me think the K scale is not linear.

• ###### 2. Re: Colour shift (from CMYK = 0 0 0 20 to Grayscale K = 22 !)

Conversion CMYK – Grayscale:

1. convert CMYK to Lab, using the ICC-profile for the specific CMYK space.

2. convert Lab to Grayscale, using the specific ICC profile for Grayscale, which is in fact

only a Tone Reproduction Curve (TRC):  L – Gray.

Thus the conversion depends on the chosen profiles.

Best regards --Gernot Hoffmann

• ###### 3. Re: Colour shift (from CMYK = 0 0 0 20 to Grayscale K = 22 !)

Hello,

Doc Maik. I see this information in the Color panel. I also looked at the Info panel after reading your post, and there, once I convert to Grayscale, I see K=22%. I was expecting for CMYK = 0 0 0 20 to convert to K = 20. Is this a wrong expectation?

G. Hoffmann. I'm afraid I don't understand the need to introduce a conversion to Lab mode in the midst of the CMYK to Grayscale conversion. What's the purpose?

• ###### 4. Re: Colour shift (from CMYK = 0 0 0 20 to Grayscale K = 22 !)

All mode changes are really profile conversions. You don't change mode - you convert to a specific profile; which can be an RGB profile, a CMYK profile, or a grayscale profile. In turn, all profile conversions go via a Profile Connection Space - usually Lab or CIE XYZ.

So there's no "need" to convert to Lab, it's just to show you what's happening under the covers anyway.

What specific numbers you end up with depends on the profile. In your case CMYK 0-0-0-20 remaps to gray 22. With a different set of profiles you'd get other numbers.

There's really no such thing as Image Mode. It's a profile conversion between whatever profiles you have as working spaces.

• ###### 5. Re: Colour shift (from CMYK = 0 0 0 20 to Grayscale K = 22 !)

Hello twenty_one,

Thanks for the clarification. Colour profile is not something I often modify hence my lack of familiarity with profile conversions. I will look into this further. For now, given that I could add the grey part of my image after the conversion to Grayscale (as opposed to before), I am adding it at that stage which allows me to get K=20 without much headache!

• ###### 6. Re: Colour shift (from CMYK = 0 0 0 20 to Grayscale K = 22 !)

Hi,

Adding some context. I am converting an image from CMYK to Grayscale inside Photoshop to use it in Illustrator. Here is what I noticed.

1- I convert the image from CMYK to Grayscale in Photoshop, via Image>Mode>Grayscale.

2- I covert the same CMYK image to Grayscale in Illustrator via Edit>Edit Colors>Convert to Grayscale.

I compare the two images (in Photoshop). I noticed a noticeable difference between the two images (the grayscale image from Illustrator is lighter compared to the same image converted in Photoshop). Looking at Color Settings in Photoshop and Illustrator, I don't see any notable difference between the two. How can I get Photoshop to do its conversion to Grayscale exactly as Illustrator did? In Illustrator, in Color Settings, I only see information for RGB and CMYK but not Gray working space.

Any help?

• ###### 7. Re: Colour shift (from CMYK = 0 0 0 20 to Grayscale K = 22 !)

Nice from Adobe to fool us this way. In the menu it's simply "Image -> Mode" not "Color profile". How on earth do we know that?! I didn't even knew that there is such thing as a grayscale profile. We know that there a profiles for CMYK and RGB, but the rest? Where to select or setup the grayscale profile in the work space?

• ###### 8. Re: Colour shift (from CMYK = 0 0 0 20 to Grayscale K = 22 !)

The need for a profile may not be obvious in this case, but somehow the C,M and Y channels would have to be converted to greyscale; this is done with a profile. The profile considers all channels including the K. Profiles apply to absolutely everything in Photoshop, all the time, so you can't expect a reminder! Since you are converting between entirely different colour spaces it is almost certain that the K will change. It's theoretically possible to derive a greyscale profile from a CMYK profile, which would preserve the K, but not clear if anyone has such a tool.

If you have an image with data only in the K channel and want this data in a greyscale - a reasonable requirement and a known weakness of profile based colour (the only kind anyone does any more), you can try making a new greyscale image and copy/pasting the K channel.

• ###### 9. Re: Colour shift (from CMYK = 0 0 0 20 to Grayscale K = 22 !)

Hi,

Doc Maik. Take my input with a pinch of salt but I think in Photoshop, you can select the grayscale profile in Edit>Color Settings.

The question mark is for Illustrator. Illustrator can convert to Grayscale but I can only find Color Profiles for RGB and CMYK in Illustrator, not Grayscale. Looking at the manual, there is the following indication:

"Grayscale also lets you convert color artwork to high-quality black-and-white artwork. In this case, Adobe Illustrator discards all color information in the original artwork; the gray levels (shades) of the converted objects represent the luminosity of the original objects."

It's not all that clear to me. Would that mean that Illustrator converts CMYK to Lab colourspace, then discards a and b to keep L only to represent K in Grayscale? In any case, I am unable to reproduce colour conversion from CMYK to Grayscale in Photoshop and Illustrator to be identical.

• ###### 10. Re: Colour shift (from CMYK = 0 0 0 20 to Grayscale K = 22 !)

There are some special rules with Illustrator/InDesign handling of grayscale images. It will be treated as K-only CMYK - which makes perfect sense going to offset press.

But Illustrator sees it as untagged CMYK - that again makes perfect sense because it does indeed not have a CMYK profile - and so Illustrator asks which CMYK profile you want to assign. This is important: not convert, but assign. This keeps numbers unchanged, but appearance changes according to profile.

To work seamlessly across this, you can set your working gray in Photoshop to "black ink - <working CMYK>", like I have done here:

This is done by choosing "load gray" in the working gray pulldown, and pick your working CMYK profile.

Note that the original question - why CMYK 0-0-0-20 turned into K 22 - is the inverse of this. Converting between profiles keeps appearance/intent, but changes the numbers to do so.

• ###### 11. Re: Colour shift (from CMYK = 0 0 0 20 to Grayscale K = 22 !)

Thanks twenty_one for your detailed reply. I read your post several times but it is not entirely clear to me. Perhaps if you can point me to a webpage or document that further expands on this, it would help me. For instance, I had not used "local gray" (black ink) in the past and don't know how it differs from Dot Gain 20%.

Nevertheless, I reproduced what you described in Photoshop. I opened an image in Photoshop, then converted the image from CMYK to Grayscale (using the Black Ink profile for the Gray Working Space), then saved the image as TIF. I placed that image in Illustrator along the same image which was converted from CMYK to Grayscale within Illustrator (first I embed the image in Illustrator, then Edit>Edit Colors>Convert to Grayscale). Comparing the two images, they look different.

• ###### 12. Re: Colour shift (from CMYK = 0 0 0 20 to Grayscale K = 22 !)

Obviously I'm using almost the same workflow as twenty_one. With full details:

1. Create a custom gray profile, derived (here) from ISOCoated-v2-eci.

This contains only the tone reproduction curve (TRC) for K-only:

This is not simply a power function of the type y=x^2.2, or a similar function as defined by Dot Gain.

At 50% we can read, as the deviation from the straight diagonal, a Dot Gain of about 16%.

At the left side the curve doesn't start at zero. It's exactly the TRC for K in a standardized ISOCoated process.

2. Convert an RGB image into Gray. This is often done by Channel Mixer, Monochrome, using the

best mixture with respect to noise and contrast. So far it is an RGB gray image.

3. Convert to Grayscale, using the Black Ink profile in the color settings for Gray.

4. Copy and paste this image (one channel) into the K-channel of an originally empty new CMYK doc

with assigned profile ISOCoated-v2-eci.

5. Place this image in InDesign. Opposed to Grayscales it's now fully color managed. InDesign doesn't

have color management for Grayscales.

6. Make a print-ready PDF by exporting uncompressed and without changing the color numbers.

I'm using PDF/X-1a. The file can contain color images in the same CMYK color space. The Output Intent

is ISOCoated-v2-eci. One profile for all graphics and images in the PDF. Not for editing but for preview.

ISOCoated-v2-eci (the spelling may be different) is here a placeholder. Twenty_one uses the same process

but with limited Total Ink of 300%.

Best regards --Gernot Hoffmann

• ###### 13. Re: Colour shift (from CMYK = 0 0 0 20 to Grayscale K = 22 !)

Adobe didn't ever offer a clean & clear workflow for handling gray images. I had critized this

since about 10 years, at that time starting the discussion 'The great Grayscale Bazaar':

'The great Grayscale Bazaar' refers to Paul Theroux:

Best regards --Gernot Hoffmann

• ###### 14. Re: Colour shift (from CMYK = 0 0 0 20 to Grayscale K = 22 !)

Hello G. Hoffmann,

Thanks for your explanations. I haven't tried your workflow yet. I see that you mention InDesign and I fear we are departing a little from my original goal which was a discrepancy in CMYK to Grayscale conversion between Photoshop and Illustrator. At present, I would like to reproduce in Photoshop the same conversion I see in Illustrator. The problem is that Illustrator's CMYK to Grayscale conversion is not clear. I followed twenty_one's instructions in Photoshop (which was aimed at mimicking the CMYK to Grayscale conversion native to Illustrator) and the resulting image was different from the same image converted to Grayscale within Illustrator.

What's interesting is that with default settings, Adobe states in both Photoshop and Illustrator's Color Settings menu: "Your Creative Suite Applications are synchronized using the same color settings for consistent color management."

By the way, twenty_one, when I change the Photoshop Gray Working Space setting to Black Ink as you recommended, this statement then reads "Unsynchronized...". I used a different CMYK working space so I loaded the Black Ink corresponding to that workspace.

In both cases, grayscale conversion don't match between Photoshop and Illustrator.

I will read the threads you pointed out (I'm more familiar with Louis Theroux's work - shame on me, you may say! )

• ###### 15. Re: Colour shift (from CMYK = 0 0 0 20 to Grayscale K = 22 !)

Actually I think Gernot is spot on. Grayscale handling is very inconsistent between Adobe apps and there is no seamless workflow from PS into Illustrator or InDesign. You have to rely on workarounds and keep your head straight.

Both Illustrator and InDesign are CMYK/offset press centric applications, and so a grayscale image should end up as K-only CMYK. That part is fine. But the conversion that we otherwise rely on isn't happening. If you don't start out with the right grayscale profile you're in trouble. The "Black Ink"-option seems fairly safe though.

What I haven't tested is whether the "gamma" grayscale profiles end up as rich black (4 color), as opposed to K-only for "dot gain" profiles. Gernot probably knows.

But one thing you should not be concerned with, is whether color settings are synchronised. Mine aren't and have never been, on purpose. That in itself is not a problem.

• ###### 16. Re: Colour shift (from CMYK = 0 0 0 20 to Grayscale K = 22 !)

twenty_one, thanks for the feedback.

Some tests show: Grayscales based on any gray profile, if converted to CMYK, deliver rich black.

This is so even for the black-ink gray profile.

It is possible to create a gray image in CMYK with K-only directly (without pasting a gray channel into the K-channel):

1. Color Settings > ISO Coated v2 (eci) > Custom

2. Choose Black Generation GCR with max.Black according to the image below.

3. Image Mode > Grayscale

The calculation is merely based on ink (here european ink), not on a previous profile. Therefore I've chosen

15% dot gain instead of the default value 9%. Thus this workflow is not entirely consequent.

Lab values are preserved, as shown. Channels CMY are empty, as required.

Printing PDF documents with RGB, CMYK, Gray and Lab components by RIP (raster image processor)

is not simple and affected by not embedded gray profiles (coming from InDesign):

http://docs-hoffmann.de/riptest05072013.pdf

Best regards --Gernot Hoffmann

Is it really not possible to edit such a post here after posting?

• ###### 17. Re: Colour shift (from CMYK = 0 0 0 20 to Grayscale K = 22 !)

Hello Gernot,

I appreciate all the input you are providing. It's a little beyond my current knowledge of colour profiles. Going back to my post regarding Illustrator (post #9), how is Illustrator deriving its K value in a CMYK to Grayscale conversion? The manual refers to Luminosity. Is that the same as Lightness in L,a,b colour system? Or is there simply no way of knowing how Illustrator is doing this conversion?

• ###### 18. Re: Colour shift (from CMYK = 0 0 0 20 to Grayscale K = 22 !)

Or is there simply no way of knowing how Illustrator is doing this conversion?

This is probably the correct answer. Nevertheless, I've actually done some tests:

Photoshop CS6: RGB = sRGB / CMYK=ISO Coated V2 (ECI) / Gray=G=2.2

Colorchecker (see Post 16, mainly the bottom row)

sRGB > CMYK > Grayscale. The result is called Image1.

Illustrator CS6: CMYK=ISOCoated V2 (ECI) / Gray=undefined

CMYK > Grayscale. The result is called Image2.

Photoshop CS6:

Open Image2 as Grayscale

Assign one after the other all kinds of gray profiles, G=1.8, G=2.2, Dot gain 10% to 30%.

Try to match the Lab values (bottom row) not by appearance but by L numbers in the Color-

checker and in the Info panel. For this comparison by numbers one doesn't have to show Image1

(but it can be used by appearance) .

Matching is impossible! Finding a match would have revealed the gray profile in Illustrator.

This kind of re-engineering Adobe algorithms is anyway not a pleasure, and the help file

for Illustrator doesn't tell us anything accurate about the conversion.

By the way, we have

Luminance = Y-value in CIE XYZ

Lightness = L-value in CIELab

Luminosity = Adobe expression (still don't know what it is colorimetrically)

[Again by the way: Illustrator CS6 doesn't keep true Lab documents as Lab. The colors are converted

into the document color space which is either RGB or CMYK (calibrated spaces = using ICC profiles).]

There are even more problems, deciphering the algorithm for a CMYK–Grayscale conversion:

CMYK is mostly created by Rendering Intent Relative Colorimetric with Black Point Compensation,

Source black L=0 or R=G=B=0 is mapped to the darkest printable black, which is between L=5

and L=20, depending on the printing process.

Source white L=100 or R=G=B=255 is mapped to paper white, which is between L=85 and L=95,

depending on the paper.

Can we anywhere expect something like Rel.Col. and BPC for Grayscale conversions? Yes, it

seems to be available in my RIP, but I don't ever use Grayscales. Where is the conceptual

problem? CMYK is always print-ready, but Grayscale is either a substitute for R=G=B gray

or a format used for printing, especially for office printers, where color management ist not

required. .

sPretzel, what exactly is your practical task? Knowing this would enable us to discuss in a more fruitful

manner...

Best regards  --Gernot Hoffmann

• ###### 19. Re: Colour shift (from CMYK = 0 0 0 20 to Grayscale K = 22 !)

Very interesting, thanks for your testing, Gernot. It appears this is a bigger mess than I thought initially.

If I ever need grayscale in the future I think I'll just paste-into-K and treat it as a CMYK file throughout. Of course you can't then convert to a different CMYK profile, because that will inevitably turn it into 4 color.

• ###### 20. Re: Colour shift (from CMYK = 0 0 0 20 to Grayscale K = 22 !)

Hi Gert,

I am preparing a file for print at a print shop. The colour space is meant to be grayscale but I am submitting a CMYK file, with only the K component (and this is fine as far as I know). I have a file in Illustrator that is meant to go in the final document. This Illustrator file includes some CMYK colour images originally. I intended to convert those images to Grayscale in Photoshop (using Mode->Grayscale!) and bringing them back into Illustrator. However, as a test to get a quick feel for what the Illustrator file will ultimately look like, I embedded the colour images in Illustrator and converted them to grayscale there (using Edit>Edit Colors>Convert to Grayscale). A cursory comparison with the same images I later converted to Grayscale in Photoshop seemed to show they looked like different greys. That's when I wrote the post.

Actually, going back in time a bit, these images have a plain colour fill background as well. In Photoshop, I first replaced that with a grey background fill (with CMYK=0 0 0 20), instead of converting the colour background to grayscale, and then converted from CMYK mode to Grayscale mode. That's where K became 22, when I was expecting 20. And this led me to investigate the entire image conversion process.

I intend to rely on Photoshop to do the colour conversion instead of Illustrator. Right now, I am planning to convert the image to Grayscale using Mode>Grayscale. I assume when I place this image back in Illustrator, the resulting ripped document will have the following components: CMYK = 0 0 0 K. However, I am wondering now if I there are better ways in Photoshop to achieve this.

----

Update:

The inclusion of a grayscale image in my Illustrator document, and then saving the Illustrator document as CMYK did not work as I thought (I thought the grayscale image would turn into CMYK= 0 0 0 K; it doesn't). It redistributed all the K to CMY. My mistake! I suppose I can add a step in Photoshop by converting the grayscale image back to CMYK before placing the image back in Illustrator. I can do that by copying and pasting the K of the grayscale into the K channel of an empty CMYK image in Photoshop.

The following threads also describe another way using GCR; I am sure these methods are well-known to Gert and twenty_one.