The following is a guest post by Carl Fleischhauer, a Digital Initiatives Project Manager in NDIIPP.
What does it mean to digitize a photographic negative? My previous pair of blogs discussed digitizing books (and other textual materials), exploring the ways that the process captures informational and artifactual aspects of the original item. The short version is that rendering the text into searchable form gets at the information, while sets of images get at artifactual values (to varying degrees, depending on the type of image).
A recent study by Don Williams, Michael Stelmach, and Steve Puglia highlights the distinction between artifactual and informational in the case of digitized photographic negatives and transparencies (city talk for things like color slides). This study was carried out on behalf of the Still Image Working Group in the Federal Agencies Digitization Guidelines Initiative.
The report tells us (forgive the oversimplification here) that you can look at a photographic negative in two ways, rather like our look at the book. On the one hand, there is the image, the picture, for which the negative can be seen as a carrier. That’s the image information. On the other hand, there is the structure (in the words of the report, microstructure) of the negative. That’s the artifact.
The classic glass or black-and-white film negative, from the 1850s forward, features an emulsion in which (after development) minute particles of silver are deposited in a pattern that creates the image. The silver molecules tend to clump a bit, producing what we call the “graininess” of the negative. This gets a little tricky: seeing graininess doesn’t mean you are seeing grains. Rather you see an optical effect caused by the clumping of the silver particles. In any case, these clumps (not the optical effect) represent the negative’s microstructure. Some of the scanners we use today could capture this structure, although that’s pushing it. You would need a microscope to see the silver at the particle or molecular level.
Memory institution scanning projects today do not try to capture the negative-as-artifact, i.e., to render detail at the level of the clumps of silver. That would require especially expensive equipment and would create cumbersomely large files. More important–as this report tells us–such capture is beyond necessity. Why is that?
Most users who seek photographic materials in our institutions fall into two categories. Some are researchers–often from academic disciplines in the humanities or fine arts–while many others are persons seeking images to illustrate a publication, museum exhibit, or television documentary. Virtually all of these users are satisfied to have the image information with no need for the artifact. They often demand very high quality (“I need to blow this up to mural size for my museum exhibit”), and they may seek the effect of graininess, but they don’t need to see the individual silver clumps. It is also the case that memory institutions retain the original photograph (often in cold storage) and patrons who want to use a microscope to see more can make a separate appointment.
So what level of resolution is needed to capture the image information from a negative? This has been a vexing problem for many of us during the last two decades of digitization projects, partly because we have not adequately thought through the distinction between informational and artifactual capture. And it is in answering this question that the report from Don, Michael, and Steve is so helpful. They state the technical goal of scanning as capturing “the full information content of the original scene.” Or to put it another way, to reach a level where increasing the resolution “will not yield any additional detail about the scene.”
How do they know when they have succeeded? We’ll hear the answer tomorrow.