Assess Your Scanning Requirements
Careful thought now will pay large future dividends. Precisely assess your scanning requirements, both now and about two years into the future. If you plan this purchase well, your scanner will do the job you want it to do now, and it will probably accommodate your anticipated needs well into the future. If you don't plan well, your scanner might not meet your present needs - too little horsepower, and it will not have the flexibility and reserve resources to meet future requirements as your business grows and changes. Plan well now, and your scanner will do what you need, and you will not have overpaid.
The Variables
Here's a list of fundamental scanner variables to keep in mind when assessing your needs. They appear in no particular order of importance--it is you who decides what's most important based on your needs and circumstances. As with other technological purchases, sometimes buying a little more machine than you think you'll need is not a bad idea. If you can confidently provide data for each of the following choice areas, you're definitely ready to buy a scanner.
1) INPUT RESOLUTION

A basic consideration for purchasing a scanner is its maximum input resolution. It has a direct impact on scan quality. The higher the resolution, the greater you can enlarge originals and still maintain high quality. Scans with insufficient resolution lack good sharpness and have rough tonal transitions. Maximum resolution typically ranges from about 600 dots per inch (DPI) for low-end flatbed to 10,000 DPI for high-end drum scanners.

To determine your needs, think about the typical kinds of media you scan and to what size you need to enlarge them. The more you enlarge a scan, the higher the resolution needs to be to not lose detail and quality. If you mostly get 35mm transparencies, chances are you enlarge them quite a bit. For example, if you enlarge a 35mm transparency to 5" X 7", that's about 525%. Assuming you'll print at 150 lines per inch, using the standard minimum resolution formula of 1.5 times the screen ruling times the number of enlargements, you will need to scan that original at 1,181 DPI. [150LPI (final screen ruling) X 1.5 (quality factor) X 5.25 (magnification)]. That same 35mm, at 1,000% (10 times) enlargement, not uncommon, would require 2,250 DPI. Scanning below this resolution will reduce the quality of the printed image. Conversely, scanning at a higher resolution does little to improve quality since the output device cannot use the extra data. The added pixels only serve to use up storage space and increase processing time.

Line art generally requires even higher resolution. If you scan line drawings, their scan resolution (assuming they are not converted to contours in a drawing program) must equal the output device resolution. This is typically about 2,450PPI or higher. If you enlarge the art, then you must multiply your resolution need by the magnification factor.

Beware of scanners that use interpolation instead of actual optical resolution when stating their maximum resolution. Optical resolution is the most important specification, because interpolation just adds additional pixels based on neighboring pixels but captures no additional detail.

2) PIXEL DEPTH (BIT DEPTH)

Pixel depth describes how many colors or tones each pixel in a bitmap can have. It's a description of how much information is recorded during the scan. In mathematical terms, 8 bit depth is represented as 28 or 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 = 256 possible shades of gray. This is the mathematics of PostScript technology. When the three colors (red, green, and blue) are mixed, the result is 16.7 million colors. This is the sum of 256 x 256 x 256. Some scanners can record more, and this allows a wider range of shadow details, although just because a given scanner records a higher bit depth, it does not mean it can differentiate more tonal levels if the scanner is of a poor design. PostScript technology will choose the 8 best bits of data regardless of the quantity created by the scanner and will discard the excess. If the scanner cannot produce data of good quality, the added bit depth does little for the image quality.

3) MAXIMUM/MINIMUM MAGNIFICATION

Make sure your scanner can accommodate your desired size of artwork. For example, if your scanner will only scan reflective art up to 6" X 8", you won't be able to scan 8" X 10" prints--among the most common sizes.

4) SCANNING MODES SUPPORTED

Make sure the scanner you buy will scan the kinds of originals you now scan and expect to scan in the next year or two. The most common modes are color transparency, color reflective, black and white reflective, and line art. With some flatbed desktop scanners, you'll need a separate, optional transparency unit to scan slides. Some scanners and software allow scanning of color and black-and-white photographic negatives, standard, but for some it is an option. If you need to scan negatives, verify this feature is included.

5) OVERALL QUALITY LEVEL

This is often a subjective topic. Image quality consists of resolution, unsharp masking, dynamic range, color accuracy, gradation. These are the things that are characteristic of a scanner. In other words, we can't change these fundamental characteristics with the software. The key image-quality characteristics of a scanner are:

Optics: Just like in cameras, scanners contain optical glass lenses and mirrors. Some optics are of better quality and design than others.
Sensing Technology: PMT scanners (drum type) still produce a wider dynamic range with more shadow detail than all but the most expensive CCD scanners (flatbeds).
USM Design: The unsharp masking circuit is critical determinant of image quality. This has a major impact on detail.
Bit Depth: Better images will be produced on scanners that record at least 8 bits of data for each primary color channel (red, green, and blue), more commonly stated as 24 bits.
Resolution: For large scans, a scanner that produces high resolution at true optical resolution will produce smoother scans with better detail.
6) DRUM VS. FLATBED
Drum-type scanners use PMTs (photomultiplier tubes) to sense RGB (red, green, and blue) values and offer very high quality scans. Drum scanners can unearth a lot of shadow detail at times not even readily visible on a transparency to the naked eye. They can reproduce a wider range in original density from a piece of art. Drum scanners generally require higher operator skill levels and require that art be flexible enough to mount around the drum. Drum scanners' maximum stated resolution applies to any original on any part of the drum at any magnification. Flatbed scanners use CCD (charged coupled device) technology to sense RGB values. The recently introduced high-priced CCD technology scanners with X/Y technology are close to PMT technology and have the added optional capability of creating "copy dot" images. The plug-ins and standalone software bundled with these devices require less-skilled operators, but result in a tradeoff with color control flexibility, desired by the experienced operator who knows far more than canned programs. An advantage of flatbed scanners is that they scan art that is on rigid substrates of any thickness, things that could not wrap around a scanner drum. A disadvantage in terms of resolution is that flatbed scanners (that do not incorporate X/Y technology) are much more limited on high-resolution scans than drum scanners for originals much larger than a 35 mm. For scanners not engineered with X/Y technology, the maximum resolution on flatbed scanners applies only to smaller originals. Resolution declines as you use more of the scanner bed. This is because on CCD scanners the reading head has a finite number of CCD elements in its horizontal array. If the original exceeds that horizontal dimension, the scanning head cannot capture all the information at its highest resolution during its pass over the art and must use a zoom lens and step down in resolution to scan the entire original. You lose pixels and resulting detail.

7) SPEED AND PRODUCTIVITY

How fast and productive must your scanner be? Is your scan volume such that you can set up and scan each original one at a time, or do you need some automation here? Most high-end drum and flatbed scanners have automated scan features that allow the operator to set parameters on several scans and then begin a scan session unattended. The scanner and software go to each original and scan according to the setups entered earlier. This is a valuable feature if you have high scan volume. If not, it might be of limited value and not worth the extra cost. Some small drum scanners only offer one drum size, and it is not removable. You can't be mounting items on an off-line mounting station while the scanner is in production as is possible with scanners with removable drums. Others have such small drums and hold so few originals, that you cannot really get any productivity increases from batch scanning software features. Scanners that require three passes (one each for red, green, and blue), can be time-consuming on large originals and high resolution settings. The three-pass scanner, typical in low-priced drum scanners, is usually designed to produce improved quality at a sacrifice of productivity. When assessing the productivity of the desktop drum scanner, watch for a slowing of drum RPM as resolution increases. If you have a high proportion of high-resolution scans, these scanners can be a poor investment.

8) SOFTWARE FEATURES AND EASE OF USE

This is a major, but often overlooked, factor when purchasing a scanner. Photoshop plug-ins (small, image-acquisition "helper" programs that scan an image directly into Adobe Photoshop) can be valuable and are usually simpler to use. Some stand-alone programs are quite robust and appeal to scanner operators who have experience with high-end scanners. They will find the gradation curves and other color adjustments quite intuitive. Conversely, the scaled-back software targeted toward low-end operators will tend to frustrate the experienced ones, who find such software lacking in control.

This lack of control and features can limit you. Where some buyers make a mistake is buying a modest desktop scanner that has few software or hardware features, and is limiting in terms of imaging flexibility and control. This creates the need for later making time-consuming refinements using an image editing program. On the other end of the spectrum, with the high-end drum scanners, the image is essentially ready to print once it's scanned. The scanner has already applied gray balance, tone correction, USM, and selective color correction. Only slight editing might be needed. The more advanced software allows you to use the default settings and only venture into complex or advanced settings as needed. Obviously, this sophistication is at the cost of capital as well as operator knowledge and skill. Your production and quality needs dictate that delicate balance between capital and labor cost.

9) PHYSICAL CHARACTERISTICS (SIZE, WEIGHT, EASE OF INSTALLATION)

Be aware of the physical characteristics of the scanners you are considering and the special working environment they might require. Smaller desktop scanners don't weigh much, and their installation is simple and straightforward. Full-size, high end drum scanners weigh hundreds of pounds, need a controlled temperature and humidity environment, and must be carefully moved and professionally installed.

10) COST

As with other technological purchases, lower-priced scanners usually have limited features, quality, and capabilities. Higher prices bring added capabilities, convenience, quality, productivity, and flexibility. Again, the key is to ascertain your needs and budget and match them with available scanners.