Excerpted from Real World Print Production with Adobe Creative Cloud by Claudia McCue.
Copyright © 2014. Used with permission of Pearson Education, Inc. and Peachpit Press.
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Appropriate Image Formats for Print
How you should save your raster images is governed largely by how you intend to use them. Often, you will be placing images into InDesign or Illustrator, so you’re limited to the formats supported by those applications. The application may be willing to let you place a wide variety of file formats, but that doesn’t necessarily serve as an endorsement of file format wonderfulness. In the olden days, the most commonly used image formats were TIFF and EPS. However, native Photoshop files (PSD) and Photoshop PDF files are much more flexible, and both formats are supported by InDesign and Illustrator. So, there’s not much reason to use other formats unless you’re handing off your images to users of other applications, such as Microsoft PowerPoint or Word.
If you need to blindly send an image out into the world, TIFF (tagged image file format) is one of the most widely supported image file formats. It’s happy being imported into Illustrator, InDesign, Microsoft Word, and even some text editors—almost any application that accepts images. The TIFF image format supports multiple layers as well as RGB and CMYK color spaces, and even allows an image to contain spot-color channels (although some applications, such as Word, do not support such nontraditional contents in a TIFF).
Some equate the acronym EPS (Encapsulated PostScript) with vector artwork, but the encapsulated part of the format’s name gives a hint about the flexibility of the format. It’s a container for artwork, and it can transport vector art, raster images, or a combination of raster and vector content. EPS is, as the name implies, PostScript in a bag (see the sidebar, “EPS: Raster or Vector?”). The historic reasons for saving an image as a Photoshop EPS were to preserve the special function of a PostScript-based vector clipping path used to silhouette an image or to preserve an image set up to image as a duotone. If you’re using InDesign and Illustrator, that’s no longer necessary.
NOTE: When you receive a JPEG image, it’s a good idea to immediately resave it as a PSD or TIFF to avoid further erosion to image content. Repeatedly opening, modifying, and resaving a JPEG can result in compromised quality if aggressive compression is used.
As applications and RIPs have progressed, you’re no longer required to save such images as Photoshop EPS. Pixel for pixel, a Photoshop native PSD is a smaller file than an equivalent EPS and offers support for clipping paths as well as duotone definitions. This doesn’t mean you need to hunt down your legacy Photoshop EPS files and resave them as PSD (unless you’re terribly bored). Just know that unless you need to accommodate someone else’s requirements, there’s no advantage to saving as Photoshop EPS now.
EPS: Raster or Vector?
It may be a bit confusing that there are raster-based EPSs (saved from an image-editing program such as Photoshop) and vector-based EPSs (saved from a vector drawing program such as Adobe Illustrator or Adobe [formerly Macromedia] FreeHand). The uninitiated sometimes think that saving an image as an EPS magically vectorizes it. Not so. Think of the EPS format as a type of container. The pixels within an EPS are no different from those in their TIFF brethren. They’re just contained and presented in a different way.
Photoshop Native (PSD)
In ancient times, the native PSD (Photoshop document) format was used solely for working files in Photoshop. Copies of those working files were flattened and saved in TIFF or EPS formats for placement in a page-layout program. While PageMaker allowed placement of native Photoshop files (yes, really—although it did not honor transparency), QuarkXPress required TIFF or EPS instead. Old habits die hard, and TIFF and EPS have long been the standard of the industry. Not that there’s anything truly wrong with that. However, Illustrator and InDesign can take advantage of the layers and transparency in Photoshop native files, eliminating the need to go back through two generations of an image to make corrections to an original file. Today, there’s no need to maintain two separate images: the working image and the finished file are now the same file.
A Photoshop PDF (Portable Document Format) contains the same pixels as a garden-variety PSD, but those pixels are encased in a PDF wrapper—it’s like the chocolate-covered cherry of file formats. A Photoshop PDF comes in handy on special occasions, because it can contain vector and type elements without rasterizing the vector content, and it allows nondestructive roundtrip editing in Photoshop.
A Photoshop EPS can contain vectors and text, but the vector content will be converted to pixels if the file is reopened in Photoshop, losing the crisp vector edge—so you lose the ability to edit text or vector content. A native Photoshop PSD can contain vector components, but page-layout programs rasterize the content. However, Photoshop PDFs preserve vector content when placed in other applications (see the table below for a feature comparison of common image formats).
Moving to Native PSD and PDF
Is there any compelling reason to continue using old-fashioned TIFFs and EPSs? It may seem adventurous to use such new-fangled files, but workflow is changing. The demarcation between photo-compositing and page layout is blurring, and designers demand more power and flexibility from software. RIPs are more robust than ever, networks are faster, and hard drives are huge. It’s still important to know the imaging challenges posed by using native files (such as transparency), and it’s wise to communicate with your printer before you embark on the all-native path. You’re still at the mercy of the equipment and processes used by the printer, and if they’re lagging a bit behind the latest software and hardware developments, you may be limited by their capabilities.
Also called “line art images,” bitmap images contain only black and white pixels, with no intermediate shades of gray. If you need to scan a signature to add to an editorial page or scan a pen and ink sketch, a bitmap scan can provide a sharp, clean image. Because of the compact nature of bitmap scans, they can be very high resolution (usually 600–1200 ppi) but still produce small file sizes (Figure 1).
Figure 1: This 1200 ppi bitmap scan prints nearly as sharply as vector art. It weighs in at less than 1 MB; a grayscale image of this size and resolution would be nearly 10 MB. Magnified to 300 percent, it may look a bit rough, but at 100 percent it’s crisp and clean.
Inappropriate Image Formats for Print
Some image formats are intended primarily for onscreen and Web use. Portable Network Graphics (PNG) images can contain RGB and indexed color as well as transparency. While PNG can be high resolution, it has no support for the CMYK color space.
s format BMP (an abbreviation for bitmap) supports color depths from one-bit (black and white, with no shades of gray) to 32-bit (millions of colors) but lacks support for CMYK. BMP is not appropriate in projects intended for print.
Graphics Interchange Format (GIF) is appropriate only for Web use because of its inherently low resolution and an indexed color palette limited to a maximum of 256 colors. Don’t use GIF for print.
JPEG (Joint Photographic Experts Group), named after the committee that created it, has an unsavory reputation in graphic arts. Just whisper “jay-peg” and watch prepress operators cringe. It is a lossy compression scheme, meaning that it discards information to make a smaller digital file. But some of the fear of JPEGs is out of proportion to the amount of damage that takes place when a JPEG is created. Assuming an image has adequate resolution, a very slight amount of initial JPEG compression doesn’t noticeably impair image quality, but aggressive compression introduces ugly rectangular artifacts, especially in detailed areas (Figure 2).
Figure 2: There’s good JPEG, and there’s bad JPEG: A. Original PSD B. JPEG saved with Maximum Quality setting C. JPEG saved with lowest quality setting
Each time you open an image, make a change, then resave the image as a JPEG, you recompress it. Prepress paranoids will shriek that you’re ruining your image, and there’s a little bit of truth to that. While it’s true that repeatedly resaving an image with low-quality compression settings would eventually visibly erode detail, the mere fact that an image has been saved as a JPEG does not render it unusable, especially if you use a minimal level of compression. Despite the reputation, JPEGs aren’t inherently evil. They can be decent graphic citizens, even capable of containing high-resolution CMYK image data.
That said, when you acquire a JPEG image from your digital camera or a stock photo service, it’s still advisable to immediately resave the image as a TIFF or PSD file to prevent further compression. However, JPEGs intended for Web use are low-resolution RGB files, inappropriate for print. If your client provides a low-resolution or aggressively compressed JPEG, there’s not much you can do to improve it. Even with the refined Intelligent Upsampling in Photoshop CC, you can only go so far. They’ll find that hard to believe, though, because they know there’s a tool in Photoshop called the Magic Wand. Good luck explaining it to them.