Microform images are commonly reduced about 25 times from the original document size. These images are generally too small to be read without some form of magnification. For special purposes, greater optical reductions may be used. All microform images may be provided as positives or negatives, more often the latter.
Three formats are common: microfilm (reels), aperture cards and microfiche (flat sheets). Microcards, a format no longer produced, were similar to microfiche, but printed on cardboard rather than photographic film.
Microforms are an economic and compact form of document representation for archival storage, but are inconvenient to read when compared with a printed book. Microform technology is used as a preservation medium, as a means of saving space (such as for the convenient storage of newspapers), or as a means of duplicating scarce or unique documents, that is, microreproductions of other original documents.
However, microform is sometimes used for original documents, for example, those created on a computer and directly printed out onto a computer-output-on-microfilm (COM) device; and for microreproductions of material assembled for the purposes of releasing an original edition in microform.
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