NOTE: This document details black and white image prep for Adobe® InDesign® or Blurb's PDF to Book workflow. For information about image prep for Blurb BookWright, see How to Prepare Black and White Images for Blurb BookWright.
The biggest challenge printing black and white images in a four-color process is the potential for the introduction of a color cast. A color cast – regardless of the type of four-color process – is emphasized when a color space is converted down from the much larger RGB color spaces (sRGB, AdobeRGB, ProPhoto, etc.) to CMYK. There is an accepted amount of color drift within the world of digital offset presses and this acceptable delta is defined by Hewlett-Packard. Most people would never notice a slight shift in Cyan within a full-color image, but a color shift in black and white images is another story.
So what to do? The best thing you can do is to be sure there is no underlying color cast in your images prior to importing into Adobe® InDesign® or your preferred book layout tool. (For BookWright look here). Here’s how:
There are several ways to convert color images to black and white using an imaging tool such as Adobe® Photoshop®. We recommend using the Black and White function that you’ll find in newer versions of Photoshop. The desaturate option can be used although it is not recommended due to its lack of control. Do not simply change the image mode to Greyscale. Because the book prints using four colors (CMYK) the greyscale mode will neither be honored nor produce the best results. The Black and White function is available only for RGB color space images. You will need to work on a RGB version of your image before converting it to CMYK.Figure 2. Black and White Adjustment
A common issue with color to black and white conversion is a loss of contrast. You can add contrast to your image by applying a slight s-curve to your images.
By using an s-shaped curve, you can improve contrast by darkening the shadows and lightening the highlights.
You can use this same technique to lighten up dark areas overall or to improve the midtone range of you images if they are too dark. Just grab the curve either in the middle or slightly to the left of middle and pull up.
You can also use the eyedroppers (circled in screenshot) to select your highlight and shadow areas of your image for better contrast control.
Your file is digitally neutral. However, you may notice that the image is flat or muddy. This is due to the nature of the desaturate command. Colors that share the same lightness value but have vastly different hues will convert to the same grayscale value. This means that you will often lose contrast between colors leading to a flat image. Newer versions of Photoshop have a Black and White adjustment that we recommend for more control and better results.
Removing all color from your image guarantees that there is no color cast present in your images. However, some photographers prefer to have a slight warm (reddish) or cold (bluish) color cast to match a traditional feel of black and white paper or sepia tone process. There is a benefit in adding a tone, as it can be easier to hold a color tone than a true neutral black and white print. There are several strategies to add a color tone to your image. We explore two of them below.
If your version of Photoshop does not have the Photo Filter feature, you can accomplish similar effects with the Curve tool.
Our Adobe InDesign Plug-in and PDF to Book workflow work best with CMYK images. CMYK images have a much higher level of control over blacks. If your images are in CMYK, you are ready to go. If you use Adobe RGB, ProPhoto RGB, sRGB, or another RGB color space, you can use Adobe Photoshop to convert your images to CMYK before placing within InDesign or your preferred layout software. Photoshop is a fully color managed application and can convert your images with a minimal amount of color shift. Make sure to always work on a copy of your image and preserve the original in its original color space.
First, if you have not done so already, download and install the Blurb ICC Profile. The ICC profile and step-by-step directions can be found in our Color Management Resource Center
Once you have made all you edits you are ready to save your files and import them into InDesign or your preferred book layout tool (other than BookWright, which requires sRGB files).
We recommend that you save a copy of your Photoshop files that have been converted to CMYK and keep your originals as they are. The CMYK files should be saved as JPG or TIFF with Embed Profile checked.
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