Digital Imaging Tutorial

3. Conversion

Key Concepts

introduction
scanning factors
rich digital master
benchmarking
text
stroke
continuous-tone halftone
proposed method guidelines

additional reading

BENCHMARKING RESOLUTION REQUIREMENTS FOR CONTINUOUS TONE DOCUMENTS

Resolution requirements for photographs and other continuous tone documents are difficult to determine because there is no obvious fixed metric for measuring detail. Detail may be defined as relatively small-scale parts of a document, but this assessment may be highly subjective. We might agree that street signs visible under magnification in a cityscape should be rendered clearly, but what about individual hairs or pores in a portrait? At the granular level, photographic media are characterized by random clusters of irregular size and shape, which can be practically meaningless or difficult to distinguish from background noise. Many institutions have avoided the issue of determining detail by basing their resolution requirements on the quality that can be obtained from prints generated at a certain size (e.g., 8 x 10-inch) from a certain film format (e.g., 35 mm, 4 x 5-inch). The important thing to remember about continuous tone documents is that tone and color reproduction is as important, if not more so, than resolution in determining image quality. See Guides to Quality in Visual Resource Imaging.

Effect of Resolution on Continuous Tone Documents: The name of the boat (Grace) is legible in the left image, which was scanned at a higher resolution.

BENCHMARKING RESOLUTION REQUIREMENTS FOR HALFTONES
Halftones are particularly difficult to capture digitally, as the screen of the halftone and the grid of the digital image often conflict, resulting in distorted images with moiré (e.g., wavy patterns). Although a number of scanners have developed special halftoning capabilities, one of the more consistent ways to scan is in grayscale at a resolution that is four times the screen ruling of the halftone. This screen ruling can be determined using a halftone screen finder, available from graphic arts supply houses. For high-end materials, such as fine art reproductions, this requirement will result in high resolutions (on the order of 700-800 dpi). For most halftones, 400 dpi, 8-bit capture is probably sufficient. Cornell did not discern any noticeable moiré when scanning a range of 19th- and early 20th-century halftones at that resolution. Lower resolutions can be used when special treatment scanning is employed. The Library of Congress has identified four distinct approaches to imaging halftone documents. See also the Cornell and Picture Elements study on imaging book illustrations for discussion of halftone treatments.

Effect of Resolution on Halftone Documents: The top image was scanned at 150 dpi, a resolution that clashed with the halftone's screen ruling of 85 lpi. The bottom image was scanned at 400 dpi and scaled for comparison purposes. Click on bottom image to view halftone grid pattern.