Building Duotones With The Proper Curves

Regular Feature: How-To's Day

Photoshop’s conversion of images into duotone, tritone and quadtone images is ironic in two ways: the default setting is not very good to print with, and the application comes with several saved settings that do the job well but are buried deep in the application files and almost never used. Here’s how to do it right!

The kicker above says it all—Photoshop has a batch of great curves included with the application that will give you sharp-looking duotones and other multitone images, but it doesn’t use them by default and duotones aren’t used that often anyway, so it’s very easy for a user to use the default without knowing that it will make shadows and dark tones clogged with ink. This is a short How-To that will show you why the defaults are lousy, where the good duotone curves are located, and how to apply them to an image. On Friday, February 17, I will follow up with an article about custom-building your own duotone, tritone and quadtone curves in order to create a great letterpress effect that has wonderful results and is easy to do.

fig1Figure 1: The original grayscale image.

DUOTONES THE PHOTOSHOP WAY

For those who don’t know, duotones, tritones and quadtones are images with two, three or four channels: a black channel plus up to three spot color channels. They’re traditionally used to improve the look of grayscale images by adding another color (and therefore a deeper and more dynamic tonal range) to the image. If you’re designing a printed piece that has one spot color, duotones can be an easy and effective way to create graphics for it.

We begin with Figure 1: a grayscale image of a sunflower. The Duotone option (in Image –> Mode) only works for grayscale images so if you have a color image be sure to convert it first. Duotones work best on grayscales with a full range of tones, including good highlights and shadows. This image is actually a little weak in the midtones, but the excess of shadow in the background will make for a better tutorial, because it’s in the shadows where the Photoshop default messes things up.

Step 1: Select Image –> Mode –> Duotone…. The Duotone Options dialog box appears. This is where you create the curves that tell Photoshop how to distribute the color.

Step 2: In the Type drop-down menu, select Duotone. You’ll see “Ink 2:” appear, with two squares beside it. The left square, which has a diagonal line through it, is where you create your curves (hence the diagonal line, which is what you see in the Curves dialog box). The right square is the color well, where you select your color for the second tone.

Step 3: Click the color well (right square) and select your color from the Color Picker dialog box that appears. For this How-To I have selected PANTONE Yellow.

At the bottom of the Duotone Options dialog box (Figure 2) you’ll see a bar that previews the range of colors with these duotone settings. It still has neutral highlight and shadow, but in the midtones you’ll see a tint where the colored ink is showing through.

fig2Figure 2: The Duotone Options dialog box.

Step 4: Save your settings if you like (click Save…), then click OK.

The problem with this method is it allows Photoshop to use its default curves, the diagonal line, for both channels, and the inks combine in unwanted ways. In the midtones, the black ink is muddying the yellow; in the shadows, the yellow and black inks are both at full strength, and the combination will probably choke any detail in the shadows. Figure 3 shows our sunflower image with the default curves, and it’s clear that the image still looks mostly “black and white” and doesn’t benefit much from the duotone process.

fig3Figure 3: The image with the default curves.

THERE’S CURVES IN THEM THAR FOLDERS

Curve combinations that work well with duotones will usually have a black curve with depressed midtones, a colored curve with shadows at less than 100%, or both. You’ll find a whole bunch of them in this folder:

Adobe Photoshop application folder –> Presets –> Duotones

In this folder are three folders—Duotones, Tritones and Quadtones—and in each of these you’ll find folders for gray/black curves, PANTONE curves and process color curves. There are 53 curve settings for PANTONE duotones alone, so that gives you an idea of the curve settings available to you! Some of these curves emphasize different parts of the value range, so I suggest you explore the various curves available, especially if you haven’t worked with duotones before. Loading the curves (by using the Load… button in the Duotone Options dialog box) and watching the results (make sure the Preview box is checked) will teach you a lot about the flexibility of duotones and what well-built duotones look like.

LET’S USE ONE NOW

Step 5: Return to the Duotone Options dialog box if you haven’t already (Image –> Mode –> Duotone…).

Step 6: Click the Load… button and select a PANTONE duotone curve setting you like for your image. For my sunflower, I like the “blue 072 bl 2″ setting file in this folder:

Adobe Photoshop application folder –> Presets –> Duotones –> Duotones –> PANTONE Duotones

fig4Figure 4: Comparing the original settings with the preset.

You’ll see a few things happen immediately. In my case, my PANTONE Yellow was replaced by PANTONE Blue 072 CVC. The diagonal curves are now true curves, and you can click on them to see the details and even edit them (which we’ll get to in a moment). As I described earlier, you can see a drastic drop in the black midtones as well as a slight drop in the blue shadows and an even slighter drop in the blue midtones. Figure 4 compares the original duotone curves (I’ve replaced the yellow with the blue for the sake of comparison) and you can really tell the difference in detail in the petals (due to the midtone changes) and depth in the shadows.

MODIFYING PRESET CURVE SETTINGS

Now that we have successfully replaced our default curves with a good set of curves out of Photoshop’s presets, let’s modify them to suit our original color!

Step 7: Return to the Duotone Options dialog box if you haven’t already (Image –> Mode –> Duotone…).

Step 8: Click the color well (right square) for the colored ink and restore it to your original color (in my case, PANTONE Blue 072 CVC becomes PANTONE Yellow). Figure 5 shows that these curves may work well for blue, but not for yellow. I’ve lost necessary midtone details and the dark background has been obliterated! We’ll edit the curves for the black ink to compensate.

fig5Figure 5: The preset curves are a disaster with the yellow!

Step 9: Click the curves (left square) for the black ink to open the Duotone Curve dialog box.

fig6Figure 6: The Duotone Curve dialog box.

This curves interface (Figure 6) is very different than the standard Curves dialog box due to the text fields shown. What’s not quickly noticed are these other quirks:

  • There are only ten editable points on the curve (0, 10, 20, 30…)
  • Points can only be dragged up or down
  • No extra points can be added

I find using the actual curve a real drag (no pun intended) so I tend to simply use the text fields to manipulate the curve.

Step 10: Alter the curve to compensate for any shortcomings the original curve has with the new spot color. In my case, the black curve needs a general increase in the midtones and three-quarter tones; you can see my final curve in Figure 7, and the resulting image in Figure 8.

fig7Figure 7: The edited curve.

fig8Figure 8: The edited curve brings midtones and shadows back to the yellow sunflower.

CONCLUSION

fig9Figure 9: From top to bottom: the original grayscale, the default duotone, the preset duotone, and the final duotone.

Figure 9 shows details of four steps during our process: the original grayscale (Figure 1), the yellow with default curves (Figure 3), the yellow with preset curves (Figure 5) and the yellow with edited preset curves (Figure 8). As the image progressed I hope you saw the development of midtones and shadows to create an image that has good contrast without sacrificing a rich tonal depth with the addition of the yellow.

Be sure to check in at Designorati.com on Friday, February 17, when I will publish my article “Using Duotone To Add Art To Your Designs”.

2 thoughts on “Building Duotones With The Proper Curves”

  1. Hi and thanks for the interesting and helpfull article.

    We do a lot of work with duotones using metalic inks as the second colour.

    As metalic inks are not transperant like other inks, do you have any advice on getting the best results.

    G Morgan
    Print production administrator
    Wollongong City Council
    NSW
    Australia
    02 42277058

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