Raster Files

Resolution, Color Mode, and File Format are the three major categories of variables when creating raster files. Attention should be given to each to insure that your job prints as you intended.

For conventionally-screened offset printing, the resolution of a picture file should be 1.5 - 2 times the screen frequency. For jobs printed on our Komori presses, we print at 175 lpi on coated stocks and at 150 lpi on uncoated stocks. For jobs printed on our Hamada press, we print at 133 lpi regardless of stock. For 175 lpi, the resolution should be between 260 and 350 dpi; at 150 lpi it should be between 225 and 300 dpi; and for 133 lpi it should be 200 and 266 dpi. The most important number here in terms of quality of output is the low end of the range. Your output quality will suffer if your resolution is too low. It won't suffer if your resolution is too high, but it will make your job more difficult to handle from an output and RIP standpoint, and excessive resolution may incur additional charges associated with excessive RIPping or processing time.

Keep in mind that these are the resolutions which should be present at the final size. If you have a raster picture that is 260 dpi in Photoshop and you place it into QuarkXPress, resizing it to 200% of the original size, the final resolution will be 1/2 of the original 260 dpi, or 130 dpi. This will be insufficient at any of our line screen rulings.

Any time you resize your photos after they've been placed, the pixels are redistributed and the resolution is changed. To find the new resolution after a raster file has been placed and scaled, multiply the original resolution by the inverse of the scaling percentage. In other words, divide 1 by the scaling percentage (127% would be 1.27, 65% would be .65, etc.) and then multiply this result by the original resolution. This will give you the new resolution. An example would be a 300 dpi photo imported to Quark and scaled to 67%. The inverse of 67% is 1.49 (because 1 / .67 = 1.49). 300 dpi multiplied by 1.49 is 447. The new resolution is 447 dpi.

If you want to know what the minimum original resolution of your file should be when you re-size at any given scaling percentage, multiply the low number in the target resolution range above by the scaling percentage. This will give you the minimum resolution which should be present in your raster file in order for the final resolution to be on target. An example would be a photo imported to Quark and scaled to 352% on a job that will print at 175 lpi. The low end of the 175 lpi range is 260. Multiply 260 by 352% and you get 915 (rounded from 915.2). 915 dpi is the minimum resolution you should have in your file at its original size.

The color mode of all files should be CMYK, Grayscale, Bitmap, or some variation of Duotone (Monotone, Duotone, Tritone, etc.). Other modes such as RGB, LAB, Indexed, or HSB generally yield unfavorable results when separated in the workflow. Convert files in these color modes to an approved color mode before placing them into your working application. See the Color Management section for more information about color at Sunset Printing.

We prefer that the format of submitted raster files be TIFF, EPS, or JPEG. Other formats may work, however these are the most reliable and can be used to achieve any printable affect. If you have a question about whether a specific file format will work, please call 503-231-8328 and speak to a pre-press technician.