To determine the proper resolution, you need two pieces of information: the final output dimensions and the resolution. (For Web, you need only the pixel dimensions and can ignore resolution.) You determine the resolution based on the image’s output device.
If the image will be printed on an inkjet printer, use an image resolution that’s one-third the printer’s rated resolution. For example, if the printer is listed at 1440×720 resolution, the image should have a resolution of 240 ppi. If the printer has a resolution of 1440×1440, the image resolution can be 480 ppi (although 240 ppi is probably fine.) You almost never need to go over 300 ppi. (Print some test images to determine what your printer is capable of doing.)
If the image will be printed commercially on an offset press through screen printing process, you need to know the line screen frequency of the press, measured in lines per inch (lpi). Multiply that number by 1.5 or 2 to find the appropriate image resolution.
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