Specifications

The Digital Fine Print Course

Creating a Fine Print

Printer Resolution

The standard units expressing digital image resolution, ppi (pixels per inch) and dpi (dots

per inch), although often confused and interchanged, do not refer to the same attribute.

• PPI (pixels per inch) is a measure of the digital file’s linear pixel resolution. The higher

the ppi, the more pixels make up the image, and therefore the larger its overall file size.

• DPI (dots per inch) describes either the size of the ink drops an inkjet printer can pro-

duce, or the number of half tone dots per linear inch that a half tone printer uses to ren-

der each halftone dot. For example, inkjet printing at 1440 dpi produces drops of ink that

are only 1/1440th of an inch (0.176 mm) in diameter. On the other hand a bit mapped

image of 200 ppi resolution printed on an image setter at 2400 dpi, will have each of its

pixels rendered as a grid of one hundred and forty four 2400 dpi sized dots (ie. 2400/200

= 12 dots per linear pixel, hence 12 x 12 =144 dots per pixel). Dots per inch (dpi) there-

fore only applies to certain types of digital and halftone printing.

Higher ppi and dpi result in finer detail in the print. However printing at 1440 dpi does

not imply that the image’s pixel resolution should also be 1440 ppi. The optimum set-

tings will depend on the pictorial and production requirements of the image. Too high a

ppi resolution will produce unnecessarily large file sizes without any visible increase in

detail in the print. Too high a dpi printer resolution will unnecessarily extend printing

times. Too low a ppi or dpi resolution will result in loss of original image detail.

The optimum resolution is easy to determine. Scan an image at various ppi resolutions at

a fixed output width and height, and print each file at the printer’s maximum dpi resolu-

tion. Observe the effect of increasing ppi on detail in the print. Then print the highest ppi

resolution file at the maximum (eg. 2880 dpi), half maximum (eg. 1440 dpi), and quarter

maximum printer resolution (eg. 720 dpi) while keeping the print width and height the

same. Critically examine each print to determine the optimum ppi and dpi resolutions.

For mural prints, a minimum resolution of a 100 ppi appears to be the visual threshold

at which most people begin to lose the ability to clearly distinguish individual image pix-

els (when viewing the print no closer than 60cm). The subjectivity in this judgement needs

to be taken into account, because in some cases higher or lower values will also be accept-

able. This doesn’t mean that the image won’t appear sharper or more detailed at higher

pixel resolutions, just that it will be difficult to distinguish pixels in the mural print. Test

print small sections from the different ppi scans to determine the optimum mural ppi.

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