Playing Fast and Loose with Scanner Specs

From DPI to dynamic range, manufacturers like to snow you with specs. Which ones really matter, and why?

Specsmanship is a time-honored game that marketing departments like to play. But there are times when they take the game so far that the specs don't mean very much.

Consider, for example, these common scanner specs that are widely misused and widely misunderstood.

DPI or PPI?

References to scanner resolution are almost always in dots per inch (dpi). The better metric is pixels per inch (ppi). It's not that dpi is wrong, but that it's too easy to confuse with printer resolution. Saying you scanned at 600 ppi and printed at 1,200 dpi is clear. Saying you scanned at 600 dpi and printed at 1,200 dpi could mean you changed the image resolution.

Resolving Detail: The Missing Spec

Scanner resolution should tell you how well the scanner resolves detail. Unfortunately, it only tells you how many pixels are in the scanned image. If the optical system of a 600-ppi scanner is limited to resolving, say, 400 ppi, the real resolution is only 400 ppi.

Dynamic Range

Dynamic range, a measure of how many different shades a scanner can see between black and white, is related to color depth, and as with color depth, it's almost always based on theoretical capabilities rather than a measure of the actual dynamic range. Here again, though, almost any scanner will have sufficient dynamic range for anything but transparencies.