More about Offset Lithography

To continue with what I covered on Friday… leaving off with halftone and separation screens… rows and rows of tiny dots to allow a press to print an image. The key thing to remember when correctly sizing  the resolution of digital images comprised of pixels (little dinky squares) is you have to know the resolution of the halftone screen. Newspapers typically print screen resolutions of 85 lines per inch to 100 lines per inch (relatively coarse dot screen). Magazines typically print in 133 to 150 lines per inch (a finer screen) and commercial custom printing ranges from 150 up to 400 lines per inch (fine screens for ultra-high image quality). Take whatever the halftone/separation line per inch resolution is and multiply that by 2 to get the pixel resolution of the raster/Photoshop image. Digital images for newspapers should be at 170 pixels per inch. Anything for magazines or most commercial printers, the digital resolution should be 300 pixels per inch.

File formats from Illustrator and Photoshop.

Print design: high resolution (300 ppi)

  • .eps (encapsulated postscript) – best for vector graphics from Illustrator.
  • .tif (tagged image file format) – best for raster images from Photoshop.

Web design: low resolution (72 ppi)

  • .jpg  (Joint Photographic Experts Group format) – lossy image compression file format for images and graphics with a full-range of color and value. The higher the compression in a jpg image, the more the image is “damaged. jpgs are a raster/pixel-based image format. Illustrator, although vector based, can save/export raster images.
  • .gif (graphic interchange format) – image format can support 256 (8 bit color) colors. Also useful in that gif can strip out unused colors to make a smaller image file. Good for flat color, limited color graphics.
  • .png (portable network graphic) – Lossless image compression technology. Images can be compressed to make smaller file sizes without image quality loss.

Other than my drawing of 6 color off-set lithography printing press, I think these two posts cover that really long lecture I gave on Friday. Sorry, but it was necessary so as to know why you do different things in InDesign, Illustator and Photoshop.

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