Graphic File Preparation for Letterpress Printing ©2016
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GRAPHIC FILE PREPARATION FOR LETTERPRESS PRINTING ©2016, Smart Set, Inc. COMMON GRAPHIC FILE FORMATS Vector Formats .ai (Adobe Illustrator) Native Illustrator file format. Best format for importing into Adobe InDesign. Illustrators’s native code is pdf, so saving files in .ai contains the portability of pdf files, but retaining all the editing capabilities of Illustrator. (AI files must be opened in the version of Illustrator that they were created in (or higher). .pdf (Adobe Portable Document Format) A vector format which embeds font and raster graphics within a self-con- tained document that can be viewed and printed (but not edited) in Adobe’s Reader freeware. All pre-press systems are in the process of transitioning from PostScript workflows to PDF workflows. Because of the ability to em- bed all associated fonts and graphics, pdf documents can be generated from most graphics software packages and can be utilized cross-platform and without having all versions of different software packages. Many large printers will now only accept pdf files for output. .eps (Encapsulated PostScript) Before Adobe created the pdf format, PostScript allowed files to be created in a device-independent format, eps files printed on a 300 dpi laser printer came out 300 dpi, the same file printed to an imagesetter would come out at 2540 dpi. PostScript files are straight code files, an Encapsulated PostScript includes a 72 dpi raster preview so that you can see what you’re working with in a layout program such as Quark XPress or InDesign. COMMON GRAPHIC FILE FORMATS Raster Formats .tiff (Tagged Image File Format) Tiffs are binary images best for raster graphics. Size is small without losing quality, they compress and transfer well, and can be imported into most layout programs. .psd (PhotoShop Document) .psd files are becoming more common, you don’t have to save out a separate version to import into the latest versions of Quark and InDesign and you can keep your layers in an application compressed format. .eps (Encapsulated PostScript / Raster) PhotoShop can also save files in EPS format, though that doesn’t mean they contain vector information. It means that they are a PostScript code file, with a 72 dpi raster header attached that gives you the preview to be able to import into a layout program. jpeg, gif, bmp, wmf, png, pcx These are formats designed for the web or for specific Windows applications. Many use lossy compression, saving in these formats can cause quality to be reduced, or color spaces to shift COLOR SPACES Bitmap A bitmap image is a one-bit binary file. Each pixel is either on (black) or off (transparent or white). Bitmap files, even at 1200 dpi, are incredibly small. Grayscale Grayscale images are 8 bits. Each pixel can be one of 255 levels between black and white. Duotone Duotones are EPS files with 2 channels, each is a grayscale level ramp corresponding to one, two, or three different spot colors. Primarily used in two color offset printing to give a toned look to photographs. Indexed Color A palette of colors that will reproduce on the web and computer screens. Not used in printing. RGB Color RGB (Red Green Blue) is the input or additive channels of white light. Input devices such as scanners, digital cameras produce RGB files. RGB (with LAB) is the largest color space and for best quality, an original RGB file should be saved before any editing or converting of color space. CMYK files converted back to RGB already have around 40% of the color information removed. CMYK Color CMYK (Cyan Magenta Yellow Black) color is the output or reductive of white light. Also know as four color process, this format is used in generally all traditional offset printing. It is the smallest, or narrowest of the color spaces, but it can be reliably duplicated with ink. Lab Color Lab stands for Luminance and a (chroma) and b (hue). This was a newer standard to develop a larger color space and create the channels based on properties, not spectrum of color. Sophisticated color editing is easier and can be done without affecting the quality of the image. This only eallyr works with a Lab scanner, which were around for a brief time and have gone away. CONVERTING RGB TO GRAYSCALE Make sure channels window is open (Window>Channels) Image>Mode>Lab Color Select a&b channels and drag them to the little trash icon, bottom right Image>Mode>Grayscale (ok to delete alpha channel) CONVERTING GRAYSCALE TO BITMAP Step One: Adjusting levels Before converting to bitmap, adjusting the levels can be the critical determining factor in how the final image looks. Image>Adjustments>Levels (or L) There are 256 levels in the histogram pictured below. By converting to bitmap, there will eventually be only 2. By sliding the outer points (white and black) in towards the middle different amounts, you can affect the overall darkness and lightness of an image and where exactly the mid point ends up. (Make sure preview is turned on. When you’re satisfied, hit ok. Remember, there’s no going back to the original once you save, so “Save as ..”) CONVERTING GRAYSCALE TO BITMAP Step Two: Set image resolution Image>Image Size The goal is to end up close to 300 dpi (ppi) at our final size. The key is utilizing the “Resample Image” button. • If “Resample Image” is NOT checked, you are simply swapping resolution for size. This is GOOD, as the quality of the image is not reduced in any way. Pictures from digital cameras will always come in at 72 dpi, but at varying sizes. First, type in 300 dpi in the resolution box. This will change the document size, but not the file size. Hopefully, the new document size is as larger or larger than your final print size. If NOT, you will need to: • Upsample your image. This does affect the quality, but can be utilized if the amount is not too severe. You will need to now CHECK the “Resample Image” button, and type in your desired final size. (Be sure to always have “Constrain Proportions” checked, unless you want to distort your image.) CONVERTING GRAYSCALE TO BITMAP Step Three: Converting to Bitmap The final step is converting from grayscale to bitmap: Image>Mode>Bitmap Input should be 300 dpi, output 1200 dpi Method should be 50% Threshold ALTERNATIVES TO 50% THRESHOLD BITMAPS Diffusion Dither Halftone Screen Good option. Currently being used with polymer plates. Used for offset, screen and flexo printing With any of these pattern/dither options, working at your final size will be important to determine the effectiveness of different resolutions, angles, frequency, etc. Pattern Dither Too regular, would probably plug up, though there’s an option for a custom pattern which could be experimented with. NOTES:.