Force CMYK Image to Black Plate?

Community Beginner ,
Apr 14, 2020 Apr 14, 2020

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Hello!

I have an image that I need to prepare in Photoshop for an offset printing workflow. It's a black and white image, currently rendered in 4 colour greys and blacks, and I need to be able to edit/convert it so that the image is still in cmyk mode, but only contains data on the black plate, with the entire image made up of k values.

I've tried converting to greyscale and back to cmyk, in hopes of discarding the cmy data, but bringing the image back into cmyk mode just re-separates the image onto all four plates. I've also tried forcing the image to the black plate with the Channel Mixer monochrome settings- it does work, but the quality of the image suffers enough that it doesn't seem worth it.

So far, my preferred method has been a workaround with InDesign. I've found that if I convert the image to greyscale, place it in InDesign, and export the indd file as a PDFx-1a with US Web Coated SWOP v2 output intent, (part of my workflow setup) the resulting image in the PDF only contains k values. (According to Acrobat's output preview). This has worked in our workflow so far, but I want to know if there's a better way to do this in Photoshop. I'd be surprised if you couldn't create a cmyk image containing only k black ... unless it's good practice to just use Acrobat or another program for converting black and white images like this. I'd appreciate any input!

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correct answers 1 Correct answer

Community Expert , Apr 25, 2020 Apr 25, 2020

Here's how you do it:

 

black-ink.png

 

Open the working gray rolldown and click "load gray". Then navigate to whatever CMYK profile you're using in the final separation.

 

Then convert your Photoshop document to this gray profile. Use Edit > Convert to Profile, not Image Mode > Grayscale.

 

Make a new document in your target CMYK profile. Click the K channel and paste your image. Empty the CMY channels by filling them with white.

 

The result is a print ready K-only CMYK file, which will output exactly as y

...

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Community Expert ,
Apr 14, 2020 Apr 14, 2020

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There are lots of ways of converting am image to grayscale in Photoshop - do an online search - then Place the image in InDesign. For colour images always work in RGB color mode and Place these in InDesign, do not covert your colour images to CMYK (unless there's a very specific reason). Export your files from InDesign to PDF/X-4 for your commercial printer (unless they have another spec).

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Community Expert ,
Apr 14, 2020 Apr 14, 2020

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One can copy/paste the grayscale version (or, as some people seemed to like, the L-Channel) into the K-Channel (edit: and fill the CMY-Channels with white), one can use a Hue/Saturation Layer set to »Colorize« and Saturation 0, … 

 

What issue do grayscale images pose in your workflow that makes you want to avoid them? 

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Community Beginner ,
Apr 25, 2020 Apr 25, 2020

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c_pfaffenbichler, can you describe to me in a little more detail how to do this? I've tried copy/pasting between channels, but couldn't get it to work- I feel like there's some picky setting or technical detail that I'm missing.

 

Also, greyscale images aren't an issue. I'm just curious if there's any way to use Photoshop to make a CMYK mode image containing only K data, (that looks good).

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Community Expert ,
Apr 25, 2020 Apr 25, 2020

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Here's how you do it:

 

black-ink.png

 

Open the working gray rolldown and click "load gray". Then navigate to whatever CMYK profile you're using in the final separation.

 

Then convert your Photoshop document to this gray profile. Use Edit > Convert to Profile, not Image Mode > Grayscale.

 

Make a new document in your target CMYK profile. Click the K channel and paste your image. Empty the CMY channels by filling them with white.

 

The result is a print ready K-only CMYK file, which will output exactly as you see it on screen. No tonal shifts.

 

Alternatively, if you're using InDesign, you can just Place your grayscale file. It will output on the black plate only when you make a press-ready PDF.

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Community Expert ,
Apr 25, 2020 Apr 25, 2020

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BTW copy/pasting between channels is just like copy/pasting everywhere else.

 

When you click a channel in the Channels panel, that channel is targeted and the others hidden. To copy, select all and copy. To paste - just paste.

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Community Beginner ,
Apr 25, 2020 Apr 25, 2020

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"From what you've said, it seems I've been doing the right thing by creating gresycale images, placing them in InDesign, and then exporting with my desired output intent. Maybe I'm fundamentally confused about how greyscale vs cmyk colour data are treated within the same file- if I understand correctly, a PDF/X-1a can contain greyscale and cmyk data, but only has one output intent. If the output intent is a cmyk profile, and you export the (InDesign) file with "Preserve Numbers", does that mean that k values from a greyscale image will be mapped to the black plate by default in the pdf?"

 

Thank you!!!

This is what I was looking for- and this appears to answer the other question that I had too. 

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New Here ,
Aug 21, 2022 Aug 21, 2022

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I have found this thread so useful, thank you for explaining things so clearly. I have had so many issues in the past with printing BW photographs using commercial printers. I am now preparing to self-publish a BW photo book on an HP Indigo 100K press, and image quality is key. My printers are recommending grayscale to avoid colour cast issues, but pure grayscale is tricky to get the blacks right. So if I understand this thread correctly, I can now print images which have been prepared as grayscale files in PS and placed in InDesign, then exported as print-ready CMYK K-channel only files. As I don't use InDesign this critical final stage has been missing, and no one has made the link between the trouble I have been having and the fact that the PS CMYK files are not recommended for BW printing! Hurrah, and thank you 🙂

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Community Expert ,
Aug 21, 2022 Aug 21, 2022

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I'm not familiar with hp indigo presses, but K-only CMYK printing has one potential shortcoming: the absolute black level. I'd recommend some test runs to determine if the blacks are black enough.

 

Sometimes it's not a problem, but for B&W photography the result may lack "punch" if the blacks aren't deep enough. Black is pretty important for the overall appearance.

 

I'm a photographer and don't know the specifics, but there are people here with pre-press experience who might know a few tricks. If it comes to that.

@Stephen_A_Marsh @c.pfaffenbichler 

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Community Expert ,
Aug 21, 2022 Aug 21, 2022

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I'm a photographer and don't know the specifics, but there are people here with pre-press experience who might know a few tricks. If it comes to that.

@Stephen_A_Marsh @c.pfaffenbichler 


 

Thanks for the shoutout, yes, the dmax or shadow value will be lower for a K only vs. CMYK in Lab L value:

 

K = Approx 8L (relative colorimetric) / 15L (absolute colorimetric)

CMYK = Approx 14L (relative colorimetric) / 18L (absolute colorimetric)

 

It is easy enough to "boost" the dark tones with supporting neutral CMY values to add density, while leaving the mid to lighter tones K only so that they are not as susceptible to grey balance shifts.

 

Apart from density, the other "issue" with K only tonal values is that the highlights will appear a little coarser as they will be comprised of a K only "stipple" rather than a CMY "rosette".

 

Here is a somewhat exaggerated visual of the difference using a 100lpi halftone simulation:

 

K-vs-CMYK.png

 

Top: K only 

Middle: Fogra39 CMYK "medium" GCR

Bottom: Fogra39 CMYK "High" GCR

 

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Community Expert ,
Aug 21, 2022 Aug 21, 2022

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You're going to get a color cast in an Indigo no matter how you send your document (color or grayscale), due to the screening algorithm. Ask the printer to run a test for you. Make a document in Lab that is Lstar 50 (A&B zero), send it that way, RGB, CMYK whatever, and ask them to print the entire sheet and examine it. 

Author “Color Management for Photographers" & "Photoshop CC Color Management/pluralsight"

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