Logos Graphics

Pre-Press for Letterpress: Image Acquisition

Posted on May 20th, 2008 by admin.

Have you ever battled raggedy edges in Adobe Photoshop? When preparing your digital files for letterpress plates, the pixelated fringe surrounding type or graphics can make your printed piece look rough and uneven. Here are some tips for transforming those jaggedy edges into smooth, professional results.

Bitmap (Raster) vs. Vector Art

First of all, let’s define the functions of standard graphic software. Many designers and letterpress printers use Adobe Photoshop to create digital files for letterpress, where ragged edges tend to appear. Why does this happen in Photoshop, and not in Illustrator or InDesign?

The answer: Photoshop is a bitmap (also referred to as raster)-based program. It uses pixels to assign and manage both color and detail information. Because Photoshop works with a fixed number of pixels, the quality of a file is dependent upon its resolution (e.g., a higher resolution file = a higher quality print). However, when type or images are modified in Photoshop, fine details are often lost or compromised — that’s when fuzzy or jagged edges can emerge.

Unlike Photoshop, Illustrator and InDesign are vector-based programs based on mathematical calculations that represent letterforms and images. Thus, type and graphics can be resized and printed while maintaining the same crisp edge quality.

Best of all, it’s a cinch to compose and format your page layout in InDesign. Within an InDesign document, you can create text blocks as well as place native Illustrator (.ai) and PDF files, all without compromising final output quality. Both Illustrator and InDesign can also generate PDF files which are convenient to show to your clients as soft proofs.

Scanning

Now that we’ve established the differences between software applications, let’s see how we can capture the best scan from your scanner.

When scanning images for letterpress, it’s best to turn sharpening off and increase the contrast between the image and its background. This prevents images from picking up small artifacts (such as white cavities in solid areas) and translates into a cleaner scan which is easier to edit in both Photoshop and Illustrator.

Here’s an example of how a scanned image looks, the first without sharpening and the second (bottom) with sharpening. Notice the small white cavities in the second image:

Using Illustrator’s Live Trace tool

Once you have scanned your image, how do you refine those uneven edges? Here’s a sample image that we scanned in grayscale mode at 350 dpi (top image); below is the same file, after it was run through Adobe Illustrator’s Live Trace tool:

Here’s how you can clean up those ragged edges:

• Open your file in Adobe Illustrator (CS3 recommended)
• Select the desired image or text using the Direct Selection tool (open arrow tool)
• Go to Object > Live Trace > Tracing Options
• Under Tracing Options, you’ll see a variety of options for our process. We’ll use the Threshold tool to determine what we keep and discard of the image as it converts the bitmap information into vector art
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• Play with different Threshold values to see how much detail you lose or hold; each image will vary. Check the Preview box to the right to see the transformed image.
• Once you are satisfied with your image, you can scale it up or down without losing any detail. However, avoid decreasing the overall image size by more than 15% to prevent loss of finest detail.
• If using InDesign, you can simply place your modified Illustrator file in its native format (.ai). Otherwise, you can use the native Illustrator file or save it as a PDF file for plate-making purposes.

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