Offset presses do not "print" using the DPI ("dots per inch") value directly; the term is actually confusing because it has several different meanings.
In your file, the correct term is PPI, or Pixels Per Inch; the number of actual pieces of data. These pixels are traditionally converted into "halftone dots" in the processes of making the plate for the press, and these are referred to in terms of LPI or Lines Per Inch; the number of actual physical spots of ink transferred to the paper.
The reason most printers ask for a 300 DPI file is that the most accurate conversion of the digital file to physical halftone uses a 2:1 sampling ratio, or 2 pixels (in each dimension) to determine the value of each halftone dot, and the average halftone screen for offset is 150 LPI (your printer who wants a 350DPI file is printing with a 175 LPI halftone screen). The next-best quality is obtained with a 1.5:1 ratio, or a 225 DPI file for a 150 LPI screen.
There's a further complication in that DPI can also refer to the actual size of the laser (or inkjet) spot used by the printing/platemaking device -- each hafltone dot is built up out of numerous laser (ink) spots. That's why you can produce smoother gradations of tone with a higher-resolution printer -- there are more laser (ink) spots available to build those dots. For example, with a 1200 dpi printer, if you print at 120 LPI, there will be 100 laser (ink) spots (10 x 10) from which to create each halftone dot, allowing for 1% increments in tone. If you printed to the same printer but specified a 240 LPI screen, you'd only be able to render 25 shades for the 5 x 5 grid of spots available.
Things are a bit different with the newer "digital presses" (essentially high-speed laser printers), which may use "stochastic screening" (randomized dots) rather than the traditional halftone screening processes, but AFAIK the same file resolutions are recommended.
Message edited by author 2009-07-04 23:36:52. |
