Adobe Photoshop CS5 for Photographers - Martin Evening [288]
CMYK previews in Proof Setup
Once the CMYK setup has been configured, you can use View Proof Setup Working CMYK to see a CMYK preview of what a photograph will look like after a CMYK conversion, while you are still editing the image in RGB mode.
If you select the ‘Dot Gain Curves’ option, you can enter custom settings for the composite or individual color plates. In the preparation of this book I was provided with precise dot gain information for the 40% and 80% ink values (these are shown on the previous page in Figure 12.36).
Figure 12.36 If you select ‘Dot Gain: Curves…’ from the CMYK setup shown in Figure 12.34, this opens the Custom Dot Gain Curves dialog. If your printer is able to provide dot gain values at certain percentages, then you can enter these here. You can make the dot gain curves the same for all channels, but since the dot gain may vary on each ink plate, you can enter dot gain values for each plate individually. Note that when you select ‘Custom Dot Gain…’ from the Grayscale workspace menu, a similar dialog appears. If you are preparing to save a color setting designed for separating prepress CMYK and grayscale files, you will want to check that the black plate dot gain setting is consistent.
Gray Component Replacement (GCR)
The default Photoshop setting is GCR, Black Generation: Medium, Black Ink Limit 100%, Total Ink Limit 300%, UCA Amount 0%. If you ask your printer what separation settings they use and they quote you these figures, you'll know they are just reading the default settings from an unconfigured Photoshop setup. They either don't know or don't want to give you an answer. The black ink limit should typically be around 95% for most separation jobs, but in the region of 85–95% for newsprint. The total ink limit should roughly be in the region of 300–350% for Sheetfed coated and Web press coated jobs, 260–300% for Sheetfed uncoated and Web uncoated jobs, and 260–280% for newsprint. If you prefer, you can just stick to using the prepress CMYK setting that most closely matches the output (such as US Sheetfed/Web Coated/Uncoated, or one of the European FOGRA settings).
Black generation
This determines how much black ink is used to produce the black and gray tonal information. A light or medium black generation setting will work best for most photographic images. I would therefore advise leaving this set to ‘Medium’ and only change the black generation if you know what you are doing.
You may be interested to know that I specifically used a maximum black generation setting to separate all the dialog boxes that appear in this book. Figure 12.37 shows a view of the Channels panel after I had separated the screen grab shown in Figure 12.35 using a Maximum black generation CMYK separation. With this separation method only the black plate is used to render the neutral gray colors. Consequently, this means that any color shift at the printing stage has no impact whatsoever on the neutrality of the gray content. I cheekily suggest you inspect other Photoshop books and judge if their panel and dialog box screen shots have reproduced as well as the ones shown in this book!
Figure 12.37 Here is a view of the Channels panel showing the four CMYK channels after I had separated the screen grab shown in Figure 12.35 using a Maximum black generation CMYK separation. Notice how all the neutral gray information is contained in the Black channel only. This is a good separation method to use for screen grabs, but not so good for other types of images.
Undercolor Addition (UCA)
Low key subjects and high quality print jobs are more suited to the use of GCR (Gray Component Replacement) with a small amount of UCA (Undercolor Addition). GCR separations remove more of the cyan, magenta and yellow ink where all three inks are used to produce a color, replacing the overlapping color with black ink. By dialing in some UCA one can add a small amount of color back into the shadows. This can be particularly useful