The guiding principles behind good file preparation (for any print process) are simplicity and clarity. Files should include everything necessary for output and omit extraneous objects, layers, guides, and swatches.
Most service providers (who you hire to output film, burn photopolymer plates, or print a job) accept InDesign and Illustrator native files by preference. PDFs can be fine as well, but only if they are fully editable: that is, if they can be opened in Illustrator and not run into font problems or missing links. Photoshop files are fine for certain purposes—line art or photos that are included in a layout, for instance—but not so good for others: it is not the best program to use for layout. Most service providers do not accept Microsoft Office or Publisher, AutoCAD, or Quark files. Service providers may have a different range of acceptable file types, so it is a good idea to check with them and find out both what they accept and what they prefer before submitting your files. To explore the various page layout programs used by graphic designers, see Software.
Imposition refers to the layout of a press sheet. Depending on the size and quantity of your piece, and the capabilities of the press on which it will be run, it may be most expedient to print it multiple times on the same sheet. Prepress operators need to make sure that the print layout will reflect the procedure and needs of their particular workflow, and every printer has layout guidelines that may differ, so if you are preparing a file for a printer, leave imposition to them.
Set your page size to the finished size of your piece. If any of your art goes to the edge of the page, turn on your software’s bleed feature and set it to .125” (9 points). In InDesign and Illustrator this can be found under File > Document Setup. In both programs bleed can also be set when you create your file; in InDesign you will have to click on “More options” in the setup dialog and in Illustrator, “Advanced”, to set bleed.
Clean Up File
Remove any unnecessary objects from your file. This includes: objects on the pasteboard that may be left over from the design process, layers that have been turned off and are not relevant to output, guides, and unused swatches in the swatch palette. Not only can these unneeded extras increase file size to no purpose, they can confuse your printer, cause delays in proofing, and introduce avoidable errors into the process. In the case of guides, it is not enough simply to make them invisible; after all, that will make your bleed invisible too; just delete them altogether. Unused swatches are similarly cumbersome; it is much better for your prepress operator to be able to glance at the swatch palette and see that the correct colors are present without having to wade through dozens of other swatches.
Don’t release art to your service provider until it is final. If you are sending it to a printer, they may want to see an in-progress version ahead of time, for pricing or just to get a sense of what technical considerations will apply, and you should certainly provide that if they ask. But when you agree to the estimate and get the ball rolling, if your art is not final it will just cause problems. Printers may have fees for change orders, so it can get expensive, but even if they don’t, having multiple files of the same piece is asking for trouble. Make your client sign off on a final version before you send it to print, if at all possible; otherwise they may try to make changes right up until and even past their own due date. (It happens!) This is especially important with rush jobs, when lots of decisions are being made in a short period and are then frequently second-guessed. That sort of thing leads to errors which there is no time to fix.
If your layout contains art from another file, such as an illustration or photo, send that file as well. Don’t embed links in Illustrator or InDesign. It may well be necessary for the printer to edit that file independently, or at least to examine it to make sure it satisfies requirements. The most common issues are resolution of raster files such as line art created in Photoshop and colors that don’t separate properly (more on this later).
Convert Type to Outlines
It is recommended to convert all type in your file to outlines. The subject of fonts is a can of worms, and font issues crop up unexpectedly and sometimes do not have a solution; the best way to avoid such problems is not to have any fonts in your file. This can be done in InDesign and Illustrator by the “Create Outlines” command, found under the Type menu. When you do this, be sure to select the text frame with the selection tool (black arrow), don’t select the type with the type tool. In the latter case, while the text itself has become outlines, the text frame is still there and still carries font information that can cause problems. Also, it makes the outlined text difficult and cumbersome to select.
Of course, there may be times when it is necessary for the type to be editable; in that case, be sure to include all fonts used in your file along with the layout file, so the printer can access them if needed. This can easily be done in InDesign by means of the Package command in the File menu, which will gather together all files associated with your InDesign file into a folder you can send to the printer. Illustrator CC (Creative Cloud, the latest version) also has this feature, but previous versions of Illustrator don’t so you will have to assemble the files manually.
Note: The use of commercial fonts is governed by licenses that are included with the font files when you purchase them. These licenses differ in what you have permission to do with the font files, so it is a good idea to familiarize yourself with their provisions before sending them to a printer. If they do not allow simultaneous use on multiple systems or transmission to a service provider, you should convert to outlines and take care of any subsequent type editing yourself.
In Illustrator, fonts are not the only items which should be converted to outlines. Objects created with symbols or brushes should also be converted to outlines. If they are not converted, the vector data they contain can change position unintentionally, is not accessible to some output devices, and it can also be impossible to separate colors properly.
Transparency is an appealing feature that is very popular these days, but should be used sparingly or not at all in file setup for print. Its primary application in letterpress printing is better served by the overprint command. The wondrous effects it is possible to create with transparency on-screen, and to a lesser extent in offset and digital printing, are not achievable practically with letterpress, where working with tints of any sort is challenging and rarely recommended. Sometimes designers use it to mimic for a client the effect of overprinting either on another ink or on a colored paper, but when it is time to submit the art to the printer, it is best to remove all transparency from the file and turn on overprinting where applicable. It is possible to see what this will look like (more or less) with Overprint Preview (in the View menu in InDeisgn and Illustrator; also available in Acrobat as Output preview in the Print Production tab of the Tools section).
Photoshop is outstanding for dealing with raster images: pixel-based art that is either continuous-tone, such as a photograph or illustration, or line art that has been scanned or drawn by hand. Because the files are raster, they are scalable to different sizes in a much more limited way than vector art such as Illustrator files. Therefore it is necessary to keep an eye on resolution and understand what kind of resolution your printer will expect in different circumstances.
The resolution required is based on the output method’s linescreen, a term that refers to the fineness of the image emitted by RIPs (raster image processors). RIPs are the brains of the output devices used by printers to create film and plates. They convert all data (vector and raster) to very high resolution raster images which are then transmitted to devices such as imagesetters and platesetters. For line art, this is very straightforward, but for continuous-tone images things get more complicated. In that case, the RIP will generate a halftone, an image that is created from parallel lines of dots in varying sizes that together comprise the image. The number of these lines of dots per inch is the linescreen, and different print processes use different linescreens for optimum output. Offset presses routinely use a linescreen, or lpi (lines per inch), of 150, 175, or 200. Letterpress plates do not hold dots as small as those in a 200-lpi image; generally, printers don’t make film for letterpress above 150 lpi.
In order to get the best image quality for a given linescreen, it is best (and easiest) to assume your image should have a resolution, commonly referred to as dpi (dots per inch), that is twice the linescreen of your output device. Thus, a 150 lpi plate would require a 300 dpi image, and a 200 lpi plate would get the best result from a 400 dpi image. It is important to remember that the resolution necessary should be based on the image at output size. If you are working with an image that is 10” x 14” and 72 dpi (the standard resolution for web-based graphics), that is the same as a 5” x 7” image with 144 dpi. If you need the image to be 300 dpi, its actual size would be only 2.4” x 3.36”. While Photoshop gets ever better at “up-sampling”, that is, generating higher-resolution image data by use of algorithms that work out what the missing data is likely to be, it is no substitute for original image data. That is why it is best to know before you create your image exactly how much resolution at output size you are going to need, so that your file contains at least that much data when you submit it to the printer. Don’t go overboard–a 2400 dpi image at 16 x 20 is completely unnecessary for a 2 x 3 output size! But if your printer has asked for 400 dpi and you send 450 to be safe, that is great.
Continuous Tone Images
A continuous-tone image is usually a photograph or illustration, and is characterized by gradual transitions between shades and colors that do not necessarily have hard edges. They should be 300-400 dpi at output size, and can be saved in different color modes depending on their output types. Full-color images can be either RGB or CMYK; output will be CMYK in most cases. RGB files are smaller (three sets of data rather than four), but since the RGB color space is larger than the CMYK color space it will not always be possible to reproduce in print what you see on screen. Because this article is meant to deal with file prep for letterpress primarily, it does not go into more detail about this subject, which is worth its own article elsewhere. Printing full color images on letterpress does not happen very often so you are not likely to need lots of specialized knowledge about it.
Images that are meant to be one color are the easiest: make sure they are set to “grayscale” (Image > Mode). You may have converted the art to black & white using a filter or layer adjustment, but if the mode is still RGB the file will be much larger than necessary and will not separate properly. If you are printing a duotone or other multi-spot-color image, use the Image > Mode > Duotone mode, which will open a dialog box where you can apply the different spot colors you need with associated curves. It is a good idea to find out exactly which swatch book you should be using so the printer will not need to go into the file and change the colors later. Letterpress is most often done on uncoated paper, so the Pantone Solid Uncoated swatch book is a good default.
Examples of line art are pen drawings, logos, and most clip art. Line art usually contains objects with very clear edges. Therefore if your file is line art, the file settings will need to be quite different from those used for continuous-tone images: the file should be 1200 dpi at output size, and the mode either grayscale or bitmap. The latter is preferable because it creates a one-bit file with very clear edges, but there are occasions where grayscale produces a better result. Bitmap files are much smaller, which is an advantage.
The other advantage of bitmap or grayscale files, apart from size, is that when they are placed in InDesign or Illustrator files it is possible to apply spot colors to them. It is much better to do it that way than to use the duotone mode to create a monotone in a spot color; colors may be changed before going to press, and it is always easier to evaluate the quality of a grayscale image than one with a pale spot color applied.
There is a good reason to request 1200 dpi for line art. The human eye is really good at spotting image errors, in this case the fact that what appears to be lines and curves are really created by blocks of pixels. When the resolution is too low, the edges of the art will show these pixels, particularly around curves, as jagged stairs. Output to film will not be clean and the resulting print will look like a mess. 1200 dpi seems to be the magic number where it is possible to fool the eye into thinking that the edges are clean, so that is the point at which the output will be reliable.
In the case of line art, it is particularly important that files be created from the start with high enough resolution. You should know the output size of your image and make sure the file contains native data at that size. That means if you are scanning art from another source, you should make sure that the scanning resolution is consistent with what you need for output. This will save you a lot of headaches later! Most scanning software offers an advanced mode where you can check settings and change them to generate the image quality you need. You can also scan from Photoshop (File > Import > Image from device) if you have a scanner attached to your computer or network.
If at all possible, don’t use Photoshop as your layout software. If you must, when submitting a Photoshop file to a printer, all type and vector art (since it is now possible to generate vector art in Photoshop) should be left intact (and that means including your font file(s) as well), and that images not be flattened. The admonition about clean files bears mention here, delete all unneeded layers! You may have built your files with different versions as you tried things out, but the printer not only doesn’t need the unused versions, they increase file size and can generate confusion and errors. Save a copy of your file that you can clean up and submit.
When submitting image files to a printer PSD and TIFF are the standard. Do not send JPEGs. JPEG is a good social file format; it is compressed and much smaller than TIFF or PSD, but it is what is known as a “lossy” format: that is, repeated saves degrade the data in the file. Also they are RGB files and are therefore unsuitable to have spot colors associated with them.