DK Intermedia | i Solutions | Printing Information
Getting Good Image Quality

With the exceptions of CD ROM photo catalogs — whose images are intended for layout position only rather than for reproduction — almost every CD ROM collection of digital photos is promoted as having high-resolution scans. But if you compare file sizes, you'll realize that's NOT the case.

• A truly high-resolution digital photo, ones that you'd find in Sports Illustrated, for example, can be anywhere from 12MB (for a 4-by-5-inch image) to 70MB (for a two-page-spread photo). The CD ROM photo collections currently on the market have files ranging in size from 500K to 22MB.
  
  
• To determine if the vendor's definition of high-resolution is sufficient for your purposes, you need to know if the resolution of the scans on the CD ROM and the line screen to be used in printing your publication.
  
  Image resolution, the amount of information in a digital file, is measured in dots per inch. Screen frequency, the density of the lines of dots used to print a continuous-tone photograph, is measured in lines per inch. Newspapers typically use a coarse line screen of 85 lpi; in-house publications, 100 to 120 lpi; and glossy magazines & good quality offset lithography, 133 to 155 lpi.
  
  
• In general, optimal image resolution is twice the line screen: optimal dpi = 2 X lpi. But size is an important factor, too. When you reduce an image, you increase the effective resolution; when you enlarge the image, you decrease the resolution. So if you're resizing the original scan for use in your publication, the formula to use is :

dpi = 2 x lpi x percent of the original size

• IMPORTANT - when dealing with huge files ( 50 - 100 Mb), you can reduce the “resolution to line screen” ratio to 1.5 without very noticeable degradation of quality. This reduction can have significant effect on final file sizes.
  
  
• Consider a CD ROM image that's 300 dpi and 5 by 7 inches. You plan to print it at 65 percent of the original size, using a line screen of 133 lpi. In this case, the optimal resolution is 173 dpi (2 X 133 X 0.65), and the 300-dpi original provides more resolution than you need to successfully print the image.
  
  
• If the CD ROM image is only 72 dpi, however, you do not have enough information in the file for good reproduction (remember that the optimal resolution is 173 dpi). The printed image is likely to lose detail, with some jagged edges where there should be smooth curves, or to appear posterized, with rough graduations in tone and unacceptable levels of contrast.
  
  
• This discussion has centered on CD ROM images, but this formula developed here is a great tool in deciding the resolution of any scan you make for use in a printed piece. In the case above, if you had several images in the piece — or many more in a larger book — scanning at 300 dpi (where 173 is all that is needed) would result in file sizes that would be much larger that necessary,wasting time on loading the files and precious disk space.
  

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