Digital imaging captures all the information in photographic originals. Read on for some guidelines on making the best digitization choices, always with the ultimate usage of the images in mind.
A high-quality standard produces versatile digital images but requires storage and manipulation of large files, while a lower quality standard generates more manageable files but often limits their utility for publication or exhibition. Selecting the appropriate quality level depends on the desired uses of the images in the long term. For instance, low-resolution images may be sufficient for classroom use by undergraduates but would be inappropriate for conservators assessing details.
Although images are often viewed on monitors, projectors, or other low-resolution devices, images should be captured and digitized at the highest quality that is practical. You can transform a high-resolution image into a lower-resolution one by reducing the image size and discarding pixel information, but it is impossible to do the reverse without pixilation. I’ve found that educating users about the importance of high-resolution images is invaluable. High-resolution master images, as well as smaller access copies of the same images, provide both the large pictures needed for reproduction, as well as quicker access to the photos.
However, digitizing at the highest possible quality is not always feasible. Full information capture creates a digital replacement of the original regarding spatial and tonal information content, which is defined by film format, emulsion type, shooting conditions, and processing techniques. Ultimately, the information content has to be determined, whether based on human perception, the physical properties of the original, or a combination of both elements before digitization.
More Details, Bigger Files
Additionally, image quality may be compromised to enhance system performance. The full quality of the image stored is often not reflected in when it is displayed, as most display devices are capable of far less resolution than printers are. The higher the quality of the image, the more storage it will occupy and the more system resources it will require, including higher bandwidth networks, more memory in workstations, and longer and costlier scanning.
Digitizing everything within a collection at high resolution and full color is uneconomical. However, if the resolution is too low, the surrogates will be a poor likeness of the image they represent.
Digital images are saved in a file format, the structure by which data is organized in a file. Despite the range of file formats, only a few are recommended for image collections. The most common formats include TIFF (Tagged Image File Format), JPEG (Joint Photographic Experts Group File Interchange Format), PNG (Portable Network Graphics), and GIF (Graphic Interchange Format). Many digital image collections use TIFF master files and JPEG derivative files.
TIFFs and JPEGs
The “tagged” in TIFF refers to the internal structure of the format, which allows for arbitrary additions, such as custom metadata fields, without affecting general compatibility. These tags describe the size of the image or define how the image data is arranged and identify the compression algorithm that is used. TIFF supports several types of image data compression, allowing an organization to select the most appropriate for their needs, and many users of TIFF opt for a lossless compression scheme such as Lempel-Ziv-Welch (LZW) encoding, to avoid degradation of image quality during compression. Users often avoid any compression at all, an option TIFF readily accommodates, to ensure that image data will be simple to decode. TIFF is the best file format for archiving high-quality images, because files may be edited and saved without damage.
JPEG is a lossy compression format that allows image data to be compressed by assigning a compromise color value to a block of pixels rather than to each separate pixel. The extent of this process can be controlled, but there is irretrievable deterioration in image quality, most noticeably in smooth gradient areas. JPEG is best used with continuous tone photographic images, destined for email and web use, or for storage when space is limited. It is not suitable for use with line drawings.
Benefits of Low-Resolution Images
Monitors display images between 72 and 100 dpi, depending on the type and quality of the display, with top models displaying 200 dpi, in which the pixels blend to the human eye, presenting a smooth image. If preparing images primarily to be displayed on the screen, such as a website, low resolution is suitable. Low-resolution images online also have the added benefit of preventing the usage of the images for commercial printing.
Low-resolution images also allow information to be shared quicker, whether emailed, posted online, or embedded in a document. Copies of the larger digital surrogates allow for ease of access.
Benefits of High-Resolution Images
Printed images require 300 dpi to replicate the sharpness of conventional photographs. Most inkjet printers are capable of producing 300 to 1200 dots per inch. When creating digital images for print, it is useful to know the printing technique and output resolution.
High-resolution images should always be 300 dpi; the size of the file depends on how large you will need the images. For example, I once directed a digital project that scanned slides from the 1960s to the early 2000s. The resulting images had to be massive—to allow users of an art history subscription database to examine the architectural details as if the users were physically there. I saved the images as master TIFFs, then duplicated them and reduced them into smaller JPEGs for access.
An increasing number of cultural heritage organizations are starting digital projects. Unfortunately, questions of image quality tied to resolution are often neglected at the beginning of such projects. The image quality choices made when the files are first created have a profound effect on cost, research value, and usefulness of the images as preservation surrogates. Image quality requirements, therefore, should be established before a digital project starts. Plan your digital project, with resolution recommendations in mind, to make sure your efforts will achieve their maximum effectiveness.