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I need to prepared B&W images for 2-color offset printing. The print company advised to use 2 × blacks (one neutral black and one slightly warm tinted black), to get really appealing blacks in offset process.

Now I need to separate the grayscale images in Photoshop to generate a duotone. I understood the way this works by setting the Pantone blacks, and defining the curves.

My question is:

How can I simulate the way it will look after printing, the same way I can do this in RGB with screen soft proofing?

Many thanks.

Arno

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  • What do you mean by "get really appealing blacks" do you need deep black? A warm one? Remove some of the screening? Banding? Do you really need a duotone?
    – Rafael
    Aug 20 at 10:38
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When working digitally with spot colors, you can't get a preview which is as trustworthy as you get when soft proofing RGB images with the proper CMYK profile.

Color management is based on measurements of actual physical CMYK test prints performed under standardized conditions. This is just four inks and the method has been perfected for decades.

When it comes to Pantone inks we have more than a thousand inks to choose from. If we just combine two inks we have a million combinations. These combinations have not been measured and as designers we normally don't have access to color profiles for spot colors.

We must rely on Adobe's quite impressive, but not completely accurate, software approximation. And our common sense and experience of course.


Part of my job is doing prepress for our 2-color offset press. On two occasions I've prepared images with a similar approach and with good results.

The first time we were to print some really artistic and dark images with lots of contrast. We chose to print with 3 inks. I made tritone images using black + black + some cool gray. One of the black channels was used to make a sort of faint base image, the cool gray was used to get more details in the light areas and the other black was restricted to the darkest areas to get really deep blacks.

The second time the images were closeup portraits with very equally distributed tones. These were made as duotone with black + some cool gray. The black was use to make a relatively complete image, but with some room for the cool gray in the light tones. The cool gray also supported the black in the darkest areas to get more depth.

On both occasions I roughly followed this method:

  • Use a couple of the actual images. (Placed them side by side in one document to see the effect on different images.)
  • Make some rough duotone curves true to the initial idea.
  • Finetune the curves to eliminate banding and get smooth transitions.
  • Compare the duotone image to the original grayscale image and adjust the curves until they look indistinguishable. (Knowing that the preview isn't completely accurate.)
  • Make a number of variations of the curves. Some slightly lighter and some slightly darker.
  • Offset print a test sheet with the variations. (A bit costly of course, but manageable as we have our own in-house press.)
  • Let the photographer choose the variation they like the most. (They will most likely want one in-between.)
  • Apply the chosen duotone curves to all the images.
  • Print with the photographer present to finetune the ink density and approve the balance before doing the whole run.
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  • The press proof I think is the crucial step here. It's definitely what I would recommend in such cases. I too was in lithographic printing for many years. You can't really see the real result of a duotone in Photoshop, or even with a good digital colour proof. Just not the same as the real thing. +1 for good general advice.
    – Billy Kerr
    Aug 19 at 23:59
  • The idea of having a cool gray+black is interesting. It is the same approach as a 5 ink, inkjet printer. What would the angles of the screening be?
    – Rafael
    Aug 20 at 10:43
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    @Rafael, we used Black and Cool Gray 7 C. Black had 45 degree angle and Cool Gray 7 had 15 degree angle. The duotone curves looked something like this. I'm not sure how the method compares to inkjet. It was all made manually.
    – Wolff
    Aug 20 at 12:02

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