Multimedia hed: Image capture dek: Turn prints or negatives into bits and bytes. by Joe Farace

While there are many ways that you can capture digital images, scanners may be the most practical for many computer users because they let you produce computer-ready files from images created with the same cameras and film you're already using. What's more, scanners can be used to digitize all the images in your existing library of prints, negatives, and slides. A few years ago, scanners were so expensive that only large design firms or service bureaus could afford them, but they're now available in a wide range of affordable prices that will fit anyone's budget--from the casual snap-shooter to the professional photographer.

How they work

Scanners convert photographs into digital form by passing a light-emitting element across the original image, transforming it into a collection of pixels gathered as a digital file. The earliest scanning devices were drum scanners, which focus a light source onto an original transparency mounted on a rotating drum. Light is aimed at mirrors and then through red, blue, and green filters that act as optical amplifiers before analog-to-digital converters turn images into ones and zeros for processing. This technology is capable of handling a wide range of image density and produces high-quality output, but the complexity of drum scanners makes them too expensive for the average computer user.

All desktop scanners use a lens to produce a digitized image. But just as all camera lenses aren't the same, the lenses (and imaging sensors) in various scanners are different, and their cost is based on the quality of the device and the type of user that is expected to use them. Just as with inexpensive film-based cameras, some low-priced scanners use plastic or inexpensive glass lenses.

Behind that lens is an image sensor--more often than not a Charged Coupled Device (CCD)--that converts light reflected off the original print into an electrical signal. The typical desktop scanner uses a CCD array consisting of several thousand CCD elements arranged in a row on a single chip. Because not all scanners can capture enough data in one pass, some use three passes. Three-pass CCD scanners use a single linear array and rotate a Red-Green-and-Blue (RGB) filter wheel in front of the lens before each of the passes are made. Single-pass scanners use three linear arrays, which are individually coated to filter red, blue, and green light. The same image data is focused onto each array simultaneously. Instead of CCDs, some inexpensive scanners use contact image sensors (CIS), which bounce light off the surface of a print onto a closely mounted sensor. CIS allows these scanners to be more compact.

Because there is such a wide range of image-capture processes, there is also a wide range of quality and price. In this article, we will attempt to guide you to the right scanner purchase depending on your needs.

Spec mix

The best way to compare different brands of scanners is to look at their color depth, resolution, and dynamic range specifications.

Color depth: The more bits you have for each color, the more realistic the final scanned image will appear, and the more expensive the scanner will be.

Resolution: Scanner resolution is specified as optical or interpolated. The maximum number of dots or pixels per inch that a scanner can digitize determines its scanner's resolution. If a scanner has a 2,550 dot sensor for scanning an 8.5-inch wide image, it has a resolution of 300dpi. In theory, a resolution of 300dpi means a scanner can resolve details up to 1/300 of an inch in the original image.

Some scanners advertise two resolutions: one for horizontal and one for vertical. To produce a 300-by-600dpi scanner, the scanner only needs to change a set of gears in order to move in increments of 1/600 or 1/1,200 of an inch in the vertical direction (back to front). When 1,200dpi or greater is selected, this process slows the scanner down, letting it pick up more detail in the vertical direction.

Dynamic range: This factor measures the scanner's tonal range. Tonal range is based on a combination of the maximum optical density that can be achieved by the hardware and the total number of bits that can be captured. Some manufacturers refer to dynamic range as dMax, and there are similarities in how it's measured practically. (In photographic terms, dMin is the brightest possible level and dMax the darkest possible level for the same image.) Dynamic range can liberally be interpreted as the range of f-stops that can be captured from a print or slide, and is rated on a narrow scale from zero to four.

Scanner types

Scanners are available in several basic types, with a few variations on those themes.

Seldom seen except at used computer stores, handheld scanners make you do all of the work by having you roll the scanning element across the face of the artwork. Handheld scanners are inexpensive, but are limited by the width of the scanner's head. Some hand scanners include software that allows you to stitch separate scans together. How well this process works depends more on the steadiness of your hand than on the software. Hand scanners formerly were the most popular types of scanner available, but as the price of desktop scanners dropped, their popularity decreased.

Snapshot scanners are a variation of the sheet-feed method to scan letters and forms for optical character recognition (OCR) purposes. This class of scanner is a compact desktop device specifically designed to scan snapshot-sized photographs. You insert a print, press a button, and the snapshot is scanned.

Flatbed scanners look like small copy machines, and there are similarities in how they work. The bewildering variety of flatbed scanners makes selection problematic. Shopping for a flatbed scanner is a matter of determining what type of images you want to digitize along with what format your final output will take. Determining the maximum size of your original images will tell you how large the glass-top scanning area must be. Next, decide the resolution of your output. For the best-quality output, make sure the resolution of the scanner exceeds your output device.

Film scanners let you scan negatives and transparencies. They are more compact than flatbeds, and their cost ultimately reflects the different film formats they can digitize. For example, 35mm scanners are less expensive than those that can digitize medium- or large-format film. One reason film scanners are popular is that they eliminate a generation by scanning the original film. Besides removing the print phase from the process, you save the cost and time associated with making a print.

The features to look for in a film scanner are similar to those of a flatbed scanner. In addition, you'll want to know what film formats it can scan. If you only shoot 35mm film or slides, you shouldn't spend more money for a larger, more expensive scanner that can handle 4-by-5-sheet film sizes. Film scanners often have an optical resolution of 2700dpi or higher because, given the small size of the original, image resolution requirements are more critical and a higher resolution scan is required to get the maximum quality out of the film. Having larger-resolution-and bigger-files also allows some cropping flexibility. In addition, a 4000dpi scan provides more pixels and greater detail of a 2700dpi scan, which results in sharper, crisper images.

There was a time when the only way you could digitize 35mm, roll, or large-format film was with a film scanner. As flatbed scanner prices dropped through the floor, manufacturers began looking for new functions to add value to their products. One is the ability to scan film in addition to prints. Using flatbed scanners to digitize film is nothing new: Transparency adapters have been available for some time as an option for many flatbed scanners. In most cases these adapters are nothing more than "light box" lids that provided a method for backlighting negatives or transparencies so they could be digitized by the moving CCD element.

In the past, this method always produced a digitized image, but the adapter was often cumbersome and always expensive--sometimes costing more than the scanner. Now that's changing. Transparency adapters have dropped in price, even becoming standard equipment for some models. Some Epson scanners offer an inexpensive (less than $100) 4-by-5 transparency unit that produces 2400dpi film scans with a dynamic rage of 3.0. Members of Agfa's DuoScan flatbed family provide a separate bed--instead of the glass on top--for digitizing 35mm, roll, or large-format film. Agfa calls this "TwinPlate" technology, and claims that it provides better control over the scanner's optical path.

Real or interpolated?

Scanners are measured by their optical as well as interpolated resolutions. Optical rez is the raw resolution produced by the hardware, while interpolated--sometimes called enhanced resolution--refers to the maximum dpi that a scanner and its software can produce for a selected image. You may see scanners that offer a maximum interpolated resolution of 9600dpi, even though the hardware itself can only produce a 600dpi image. When the resolution specified exceeds the optical resolution, software interpolates the data by resampling the existing data using mathematical formulas to fill in the area between existing pixels.

If you're wondering if an interpolated scan is as good as one made on a scanner with the same optical resolution, the answer is maybe. At two times the optical resolution, any quality differences will probably not be noticeable for most color photographs. If a scanner has an optical resolution of 600dpi, you should be able to make scans at 1200dpi with minimal loss of image quality.

Anything higher than that, and you are playing resolution roulette. One way to improve an interpolated scan's appearance is by using the Unsharp Mask command found in Adobe Photoshop and other image-editing programs. This oddly named function is a digital implementation of a traditional darkroom technique in which a blurred film negative is combined with the original to highlight a photograph's edges. In digital form, it's a controllable method for sharpening an image. Sharpening the image using Unsharp Mask can add edge definition and crispness to an interpolated image.

Ease of use

Even if a scanner is easy to use, it might not give you usable results. One of the most important developments in scanning technology was the development of the TWAIN standard. It's not an acronym, although some pundits claim it stands for "Technology Without An Interesting Name." TWAIN devices allow compatible software to scan text or images directly from within an image-editing application. Scanning software is packaged with all flatbed and film scanners, and the manufacturer's goal is to make the scanning process as simple as possible.

Most scanner manufacturers bundle proprietary software to allow you to scan images with the least amount of muss and fuss. Agfa's ScanWise software, for example, scans originals and then inserts them directly into your chosen application, allowing you to scan directly to your word processor, image editor, Web browser, fax, or e-mail application. Its auto-processing feature rotates misaligned originals and checks to see if they're text, color, or black and white.

The right connections

There are several ways to connect a scanner to a computer: You can connect via Small Computer Standard Interface (SCSI), Enhanced Parallel Port (EPP)--often just called parallel--or the Universal Serial Bus (USB). SCSI connections used to be standard equipment on all Mac OS computers, although that's no longer true. Parallel ports are still found on many Windows computers, usually as a printer connection. Most parallel-port scanners have a "pass through" that lets you connect both printer and scanner. Right now the most popular connection standard is USB, which was designed to eliminate the cable clutter often found dangling out the back of many computer systems and to reduce incompatibilities between peripherals. You can connect or disconnect a USB device while the computer and scanner are turned on, something that spells disaster for a SCSI device. Recent versions of the Mac OS and Windows 98 and later support USB, and most scanner companies produce inexpensive USB-equipped models.

Like traditional image making, digital imaging is part of a process that only begins with capturing the image. That complete process starts with capture but also includes manipulation, output, and presentation. Of these, the capture phase is important because, following the old computer adage of "garbage in, garbage out," the higher the quality of the digitized image, the higher the quality the output image will be. Needless to say, serious graphic artists need to get the best scanner they can afford.

Contributing Editor Joe Farace is author of more than 20 books on photography and digital imaging.