Many of the images we create on computers spring directly from our imaginations onto the screen, but most, for better or worse, depend on capturing an original image from outside the computer. Although there was a time when art was captured by tracing over an original with the stylus of a digitizing tablet or using video cameras and video capture, the primary tool for bringing such images into the computer art system today is the scanner.
Computer artists use scanners for a number of tasks. The three most popular uses are scanning photographs for direct printing, scanning line art for direct printing, and scanning line art for conversion to vector-format images via autotracing programs.
Scanners come with a great range of capabilities and prices. Unfortunately, many of the most loudly hailed features exist only for the purpose of writing advertising copy. So, choosing the right scanner can be confusing.
Grayscale flatbed scanners usually provide good full-sized images up to 8« x 14 in., but they also limit your ability to perform the full range of scanning functions and negate any cost savings. If you want to use the scanner strictly for scanning photographs that will be printed as halftones, this is not such a great concern. However, if you plan to convert the scans to vector art with Streamline or a similar autotracing tool, or hope to extract the best line art out of marginal-quality originals, the ability to separate image colors gives you more options. In addition, if you are attempting to pick up an image that was previously printed in color on colored stock, the contrast available in a color scan is often much greater than the contrast between two shades of gray.
Drum scanners, which provide a very high resolution and dynamic color range, start at over $10,000. Obviously, it is much better to turn to a service bureau when you really need the quality.
So, what is the most practical type of scanner? For virtually every art department, the ideal choice is a color flatbed scanner. Common flatbed scanners offer scanning resolutions of 400-600 samples per inch (spi) and scanning areas of approximately 8« x 14 in.
Because the maximum number of samples, and thus the file size, increase with the square of the scanner's resolution, the time and disk space necessary to handle higher-resolution scans quickly become prohibitive. (A 200 spi scan is four times as large as a 100 spi scan.) Frequent scanner users quickly learn to scan only the necessary information at the lowest resolution that will provide a quality image. This resolution is only rarely higher than 300 or 400 spi for continuous-tone originals.
Many scanners are also advertised in terms of their interpolated resolution, which means that they include software that can cleverly generate values between the actual values that the scanner's Charge Coupled Device (CCD) array reads. Is this an advantage? Not really. Your image editing software (Photoshop, etc.) or autotracing software can provide the same functions without ballooning the file sizes.
Various experts have determined diverse proportions between scanning and halftone resolutions, but most agree that the maximum scanning resolutions are 200% of the halftone resolution. I have spent many hours analyzing this relationship, and I have concluded that the scanning resolution needs to be 141% of the halftone screen's resolution. To allow for possible last-minute changes in size, I tend to scan at about 160% of the halftone resolution. This means that to create a same-size image to be reproduced at 65 lines per inch, you should scan at a resolution between 92 spi (my minimum) and 130 spi (consensus for the maximum necessary). Even if you were to print the image at three times the size of the original photograph, this suggests an absolute maximum scanning resolution of 390 spi.
In any of these calculations, only the true hardware resolution is significant. No more information can be created by interpolation, so scanning at the higher values is mostly a delusion.
Scanning speed is another important concern. It won't seem as important to someone who currently goes to the service bureau for a scan, but the difference between one minute and five minutes becomes significant if you use your scanner often. Plus, if a scan takes less than a minute, you won't hesitate to rescan something that isn't quite right.
A related consideration involves the choice between single-pass and three-pass scanners. A three-pass scanner makes a separate pass over the scanning area for each of the three colors, which may take longer than a single-pass scanner and can potentially lead to misregistered color. However, performance and quality differences between the two scanner styles are no longer great. I still prefer single-pass units, but that is mostly a holdover from a couple of poor three-pass designs that are now five years old.
Adherence to industry standards may not be important right after you buy a scanner, but it becomes critical as your needs become more sophisticated. Scanners for the PC should operate with standard SCSI hosts, and all should definitely comply with the TWAIN standard. (TWAIN is a scanner interface developed primarily by Hewlett Packard. Believe it or not, the acronym stands for Technology Without An Interesting Name.)
A Scanner should also come with a plug-in module so it can be operated from within image-editing programs like Photoshop. It should probably have a standalone scanning module as well. Even if you do not intend to use Photoshop, more and more image-editing applications are being written and Photoshop-style plug-in modules will help you remain flexible. A scanner complying with these standards will certainly run with software that you may acquire in the future.
The "bit depth" of the scanner is something new to watch for. A year or so ago, all the common scanners read 8 bits per color. Several manufacturers are now building scanners that read 10-12 bits per color, and newer image-editing software is beginning to be able to use the additional information. (Photoshop 3.0, which will be available about the time you read this, will manipulate 12-bit-per-color files.)
The image-editing software that is bundled with the scanner should also be taken into account. Although many scanners are now being shipped with fully functional copies of Photoshop or Photostyler, less expensive scanners often come with limited versions of these programs, or with programs that are less capable to start with. If you will be dealing with photographic images, you are going to need the full version of these image-editing programs, so don't consider limited software a valuable part of the package.
Unfortunately, most computer stores do not stock a wide variety of scanners that you can try. Since the purchase price of a new scanner generally ranges between $800-3500, it isn't wise to buy one without a test drive. This is one of those decisions where contacting other artists in your community or in the SPAI could also make a profound difference. My experience over the last couple of years leaves me very comfortable with Hewlett Packard, Microtek, and UMAX scanners. If you choose another brand, I'd love to hear how your images come out.