Excerpted from THE KODAK WORKSHOP SERIES: DIGITAL PHOTOGRAPHY
by Jenni Bidner. Copyright (c) Jenni Bidner, 2000. Published by Silver Pixel Press
(New York). Used by arrangement with Silver Pixel Press (New York).
Once you’ve decided on the type of scanner you’re interested in, it’s time to look
at the details. It’s tempting to randomly select one based entirely on the price
you have in mind. But unless you’re lucky, you won’t come out with the best
scanner for your needs.
Since technological capabilities for scanners keep improving, what constitutes "poor," "adequate," and "great" changes along with the technological jumps. Hence to make a good purchase decision, it becomes important to understand the underlying factors that differentiate one scanner from another..
Optical resolution is often the lead specification in scanner advertisements. After all, it’s usually the first concern—as well as one of the more confusing factors—for newcomers. If it were the only factor (or even the most reliable clue) as to how the scanner will perform, then shopping for a scanner would take about as long as it takes to dial a phone. It is, however, a good start.
Optical resolution is measured in dots per inch (dpi), and the higher the resolution the scanner offers; the larger you can successfully enlarge your scan for any given application.
In order to decide at what dpi you need to scan your originals, you have to think ahead to your end application. A 300 dpi flatbed scanner isn’t what you need if you have professional publishing aspirations, unless you’re planning to do a pocketsize book with tiny pictures. But it’s great if you just want a scanner for fun and games— like e-mailing pictures, small output to an ink-jet or other color printer, children’s artwork, or optical character recognition (which only requires 250 dpi or so). A 2400 dpi unit would be overkill in this case. However, if you hope to manipulate your digital images, output them, and get photo-realistic quality prints of a reasonable size, you’re
going to need more powerful optical resolution (not to mention 24-bit color or better). Likewise, the successful scanning of line art (including signatures) requires very high resolution scans to achieve smooth lines without jagged edges.
Interpolated or Enhanced Resolution
One misleading specification is interpolated or enhanced resolution. Interpolated resolution starts with a hardware optical scan at a lower resolution, followed by a software guess at what the higher interpolated resolution should look like. In other words, it bumps up the dpi and estimates the color of the added pixels. Some software guesses are better than others. Regardless, it’s always better to have the real optical reading, so judge your scanner by its optical claims.
Interpolation, however, can be good in another situation. When the scanner is being used at less than its maximum dpi capability, some scanners simply skip readings. For example, they just make every other reading when scanning at 600 dpi on a 1200 dpi scanner. This enables the scanning mechanism to cover the distance twice as fast and complete the scan faster. However, most experts will agree that the best results occur when the scanner takes the maximum reading (1200 dpi) and then uses its computing power to interpolate the data downward rather than just drop every other or every third pixel. This can yield smoother tones and gradations. If your scanner can’t achieve this, for top-quality work scan at the maximum setting and then use a professional photo-editing software program to perform the downsizing. In the case of halving the file size, side-by-side pixels would be evaluated to come up with one new pixel value.
If your scanner doesn’t capture the subtle differentiations between grays (in black-and-white scans) and between colors (in color scans), then this information from the original is lost. This differentiation ability is called color depth or bit depth. (See the second chapter for more details on bits and bytes.).
In black-and-white photography, a print with 256 shades of gray looks "photographic." It is
smooth and seamless to our eyes. But 256 colors (8-bit color) appear posterized and fake when viewing a color image. The gradation of colors is jumpy. If we give 256 steps (8 bits) to each of the RGB colors that create the color palette, these colors can be mixed in 16.7 million ways (256 x 256 x 256) to create 16.7 million color choices. Thus 24-bit color (8-bit red plus 8-bit green plus 8-bit blue) looks real to us. With a 30-bit or greater system, the extra information can be used for even more colors, or more likely, for advanced functions like creating color information that can be exchanged for noise or other damaged information bits to create a final, better 24-bit package.
In other words, it lets the scanner throw away bad or corrupted information, leaving only the best 24-bit information. If you have a black-and-white monitor, a black-and-white printer, and no aspirations of using your digitized images in any color applications, an 8-bit system that delivers 256 shades of gray is more than adequate. But for more versatility, you’ll probably want a color scanner, which can, of course, also scan in black and white.
Being able to differentiate among subtle gradations of tones is a much-overlooked factor that can have a huge effect on the printed outcome. If you understand the science of photography, you know the importance of the density range of films and printing papers. The more tones the film or print can potentially show, the more realistic the image will look. Professional conventional photographers still work long and hard to pick the best film, lighting, exposure, and developing combination to give them the tonal variations they hope for in their images. Usually the goal is a large or long density range that holds as much detail as possible in the highlights and shadows. In the digital realm, we strive for the same thing, i.e., a higher dynamic range.
The best scanners are able to differentiate among subtle gradations of tones, giving them a higher dynamic range, which is rated on a logarithmic scale from 0.0 (low) to 4.0 (high). It’s no surprise that these scanners are considerably more expensive than scanners with a short dynamic range, which could have trouble giving you good scans of transparencies or negatives that have high overall contrast. If you need to reproduce the whole range of the original, with the ability to distinguish among subtle tones in deep shadows or bright highlights, hold out for a scanner that has a density range of 3.5 and above, or use a service provider.
If you’re scanning color prints, you need not worry as much. Most color papers have already flattened out the density range to about 3.0 before starting. This is one of the reasons so many professionals use transparency films.
Drivers and Plug-Ins
Drivers and plug-ins are all about simplicity and, therefore, should not be ignored. Drivers enable the scanner (or other peripherals) to communicate with the computer. Scanning times claimed by the manufacturers may not live up to your expectations if the driver is poor. Plug-ins allow the scanner to be used with the most popular graphic design and photo software, offering easy setup and integration.
A relatively new technology called Digital ICE (from Applied Science Fiction Inc.) has recently been added to some scanners. Digital ICE automatically removes surface and near-surface defects from scanned images without altering the underlying image. This can save considerable time by eliminating the need for painstaking retouching of dust spots and scratches from your scanned negatives, transparencies, and photographic prints. The newest generation can even restore colors on faded images, automatically.
Having said that, let's also say that you are now in position to get some great pictures, like this:
Sometimes your scanner choice may be more dependent on special accessories or functions than anything else. If you’re going to scan the 20,000 slides in your stock file, a dedicated desktop film scanner with an accessory 50-slide feeder could save you days of labor.
Likewise, the ability to easily batch images on a flatbed scanner might be important. Almost any flatbed scanner will allow you to place several at one time (or twenty 35mm slides if it has a transparency adapter) simply because they fit on the glass. But can the software treat them as separate images for easy saving as separate picture files? Being able to save the scanned image in different picture file formats might seem minor, but it can be a major headache if it doesn’t work. Make certain the scanning software (or plug-in for the photo software) allows you to easily save your scanned images in the file formats you most often use. It will save you time and eliminate translation problems when later sharing the images.
Other considerations are specific to the occupation. Can the scanner accommodate chest X-rays or mounted dental X-rays? Are there tethered fiber-optic extensions for remote scanning? Can you program multiple profiles for different scanning functions or multiple users? Read your industry trade journals to find out about the latest and greatest scanners for your particular field of expertise.
Line Art Black-and-white line art is any image or graphic made up of two tones— black and white or blue and white, etc. Your signature is line art, as are many monochrome logos. Surprisingly, scanning line art requires relatively high resolutions to achieve good results. If you scan under 1200 dpi, your results may not be smooth and sharp. Jagged edges or "jaggies" are the most common problem that occurs from line art scanned at too low a resolution.
Other Factors Read the specifications carefully before buying your scanner. As mentioned earlier, enhanced or interpolated dpi can be misleading. Likewise, your scanner may not match the manufacturer’s claims if your computer’s processing speeds are slow or if too little computer RAM has been allocated to the scanner. Software bundling should be factored into the price of your scanner only if you need the software being offered. Note that many of the bundles are often scaled down, less powerful versions of big-name software programs. Popular bundling packages can include photo-editing software, OCR software, and software that gives the scanner photocopying or fax capability.
Pick your scanner based mostly on what you plan to do, and then add in a little extra power for expansion and growth. But the best advice is not to drastically overbuy. You can always farm out the occasional piece that can’t be handled by the machine you select. At the rate prices are falling and capabilities are increasing, the difference in price between the scanner you need now and the higher-end scanner might be the entire price of the better scanner in two years.
Yet be careful not to underestimate. The new generation of home-use computers can easily handle FlashPix image files. Near-and true-photo-quality printers are dropping in price so quickly that you may not be able to resist adding one to your collection. These printers handle a lot higher resolution and wider dynamic range than your older (and possibly archaic) 300 dpi ink-jet. Even if you don’t splurge for a photo-quality printer, service providers can print your high-resolution digitally enhanced photos with high-resolution printers, including units that print onto silver-halide paper. So analyze what you honestly aspire to do, as well as what you currently need to do.