Archived posting to the Leica Users Group, 2003/06/15
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]I'm going to take a stab at this. I work for a commercial printer in Ottawa, and have lived the challenges of prepress and image size. I 've created a small spreadsheet that calculates image size based on line screen. The rule of thumb in the press world is that the DPI rating of an image should be about twice the halftone line screen (LPI) used to generate film/plates. In the regular commercial offset world, halftone screens are generally 150-175 LPI. Therefore any image used should be 300 to 350 DPI. In the newsprint land, because of the presses and stock used, line screens are generally 85-100 LPI. The corresponding image DPI should be in the 150-200 DPI range. There is considerable latitude in this rule, primarily based on the image. Softer images are generally more forgiving. One thing I always remind my less technically savy clients that the DPI is the final -- as opposed to starting -- resolution. I have had clients who start with a 4 x 5 300 DPI image, and enlarge it 200% in Quark. Doesn't work! I don't work with digital cameras (yet) but this should give some help to those need to work with digital images and the print world. If I'm off track on any this, I'd appreciate some friendly correction! Here are some sample image size calculations from my spreadsheet. I checked these calculations by creating dummy PhotoPaint documents, and they seem to be in the ball park, for TIFF images at least. JPGs are completely different beasts with the lossy compression settings, and native CPT or PSD files have lots of other factors affecting file sizes. Offset, B&W, 150 LPI halftone height: 9 inches width: 6 inches area: 54 square inches dpi: 300 total pixels: 4,860,000 colour depth: 8 bits (256 grays) image size, bits: 38,880,000 image size, bytes: 4,860,000, or 4.9MB (note: one byte = 8 bits) Offset, colour, 150 LPI halftone height: 9 inches width: 6 inches area: 54 square inches dpi: 300 total pixels: 4,860,000 colour depth: 24 bits (RGB) image size, bits: 116,640,000 image size, bytes: 14,580,000 or 14.6MB Newsprint, B&W, 100 LPI halftone height: 9 inches width: 6 inches area: 54 square inches dpi: 200 total pixels: 2,160,000 colour depth: 8 bits image size, bits: 17,280,000 image size, bytes: 2,160,000 or 2.2MB Newsprint, colour, 100 LPI halftone height: 9 inches width: 6 inches area: 54 square inches dpi: 200 total pixels: 2,160,000 colour depth 24 image size, bits 51,840,000 image size, bytes 6,480,000 or 6.5MB - -- To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html