0 Replies Latest reply on Apr 9, 2002 8:48 AM by LenHewitt

    Image Resolution versus Line Screen Frequency

    LenHewitt Level 1
      >> I've always heard the "rule-of-thumb" number as ranging from 1.5 to 2.0x<br />the linescreen frequency of the output device. <<<br /><br />This little bit of conventional wisdom is so firmly embedded in the<br />consciousness of the industry that I sometimes despair of *ever* rooting it<br />out. But I continue to try. <g><br /><br />The easiest place to find quick reference on this is in the January 1994<br />issue of Pre magazine in a column by Brian Lawler. (<br />http://www.thelawlers.com/FTP/Resolution%20Essay.pdf.sit ) But it has also be<br />covered elsewhere, among other places, in Publish, in the July 1992 issue in<br />an article written by Jane Hundertmark about the process the San Francisco<br />Examiner went through in switching their production over to a digital<br />process.<br /><br />To keep it simple, it goes like this. When you halftone an image, the *best*<br />resolution you can get is the frequency of the halftone screen. So if you<br />use a 1:1 oversampling ratio, your image resolution, (at 1:1), should be<br />that of the halftone screen. 100, 120, 130, 150 ppi. HOWEVER, this is only<br />going to work really well if the halftone screen is aligned with the pixel<br />grid. If it's turned at an angle, you'll get dropouts and really bad<br />averaging. So you need to get between 1.16 and 1.18 pixels per dot as a<br />minimum to get good averaging and to compensate for screen angles. You can<br />improve the quality of output on<br />any image to an oversamping ratio of about 1.25:1. Problem images,<br />(herringbone tweed, for example), can require as much as 1.4:1. Beyond that,<br />nobody can tell the difference.<br /><br />But don't take my word for it. Try it for yourself.<br /><br />"Bruce Fraser and David Blatner used the "standard 2x" rule of thumb in RWPS3."<br /><br /> You're right, they did. But take a look at the images illustrating concepts in oversampling in Blattner and Roth's _Real World Scanning and Halftones_. They are resolved at 186ppi. 1.4 times a 133 lpi<br />screen. <very wide grin>