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Effects of Scan Resolution on Print Quality

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This is a print sample of the target image. The image was scanned 4 times at 100, 200, 300 and 400 ppi. The test sheet was printed on photo paper on an Epson Stylus 740 color printer in High Resolution mode (1440 x 720 dpi). Look at the enlargements to the eye area below to see the differences.

The 100 dpi print shows severe pixelation over the entire image. At 200 dpi the pixelation is almost gone but can still be seen in the reflections in the eye and particularly along the eyelids.At 100 and 200 dpi the highlights in the hair do not have the same tonal range as in the higher resolution prints. At 300 and 400 dpi there is no pixelation and the differences between the two (even with a jewelers loupe)is impossible to detect. The primary difference is in file size. The 400 dpi file is almost twice as large as the 300 dpi file (1.92 meg compared to 1.08 meg)

Optimum resolution for printing ( rule of thumb) is 1/3 printer resolution. The idea behind this is that the printer prints 3 dots ( cyan, magenta, and yellow ) for each pixel. Therefore, 240 ppi ( pixels per inch for image resolution) times 3 (dots per pixel) equals 720 dpi ( printer resolution ). A 600 dpi printers optimum resolution would be 200 ppi.

Through personal observation I have found that an image of 300 ppi produces a better print image than the "optimum" 240 ppi image. The printer people I've spoken to explained that even though the printer driver throws out the excess information it uses it when building the image and provides better overall tonal quality. Try it our yourself.

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