Digital Media Services

Slide Scanning: A Sticky Wicket

Digital Media Services (DMS) has helped numerous faculty and staff digitize their collections of 35mm slides during the last two years of operation, scanning more than 2,500 slides on users' behalf in 2002 alone. While 35 mm slides have been the medium of choice for many photographers and teachers because they are portable and durable, they are notoriously difficult to scan. Many libraries and museums are now struggling with large slide collections, and trying to determine whether and how to convert them to digitized form. Northwestern's Art History Slide Library contains more than 250,000 slides, mostly 35mm and in glass mounts.

Many commercial photo-processing houses can turn slides into prints, negatives or even Kodak PhotoCDs. Depending on intended use, PhotoCD may be a good option for painlessly acquiring several digital versions (at varying resolutions up to about 4,000 by 6,00 pixels) of a slide. However, the Kodak PhotoCD system does throw away some of the information about each digital image through compression, and therefore may not be an acceptable choice for long-term or archival backups of physical slides. Whether or not the PhotoCD system is used, it is a good idea to hold on to original slides as long as they are in reasonably good physical condition.

Resolution

Resolution is an important aspect of any scanning project, but is particularly important for 35mm slide and film scanning because the originals are so small. In order to achieve a 300 dpi (dots per inch) 3'" x 5" print, a 1" x 1.5" slide must be scanned at a resolution of at least 1,000 dpi! The resolution equation becomes more complex when the size of the original is taken into account. If the purpose of scanning a slide is to be able to reproduce a print equal in size to the work originally photographed, sometimes a resolution much higher than 1,000 must be used (see figure 1). Many mid-range slide scanners (in the $1,000 - $3,000 range) support resolutions up to about 4,000 dpi.