The lens determines the quality of the images that are formed inside a camera. The lens and its quality determine what kind of image your camera can collect and to a great degree set the ultimate quality of picture you can hope to take.

You can take photographs without a lens—that’s the principle behind pinhole photography. But nearly all cameras have lenses for one very good reason: A lens collects a lot of light. It captures more photons. That gives the camera more light to work with and makes exposures quicker. The lens also affects the view, what the camera sees. It even alters the aesthetics of the image you make.

Aperture

Literally speaking, an aperture is nothing more than a hole, and that’s what it means for cameras. The aperture is the hole in the lens through which light can get to the image sensor. In photography, digital or otherwise, the aperture is more important than a mere hole. It’s a variable hole, one you can make larger or smaller to let more or less light reach the film or sensor. A larger aperture lets more light in; smaller lets in less.

Varying the size of the aperture helps a camera of any kind cope with light conditions. When light is too bright, it can overwhelm the image sensor; too dim, and the image sensor might not be able to find enough photons to make an image. To prevent these problems, most cameras use wider apertures in dim light and smaller apertures in bright light.

The mechanism for setting the aperture is termed the iris of the camera, and it corresponds to the iris of the human eye. By sliding thin plates called iris blades—in manual mode typically by rotating a ring around the lens termed, appropriately enough, the aperture ring—you can adjust the size of the hole between the blades and thus the aperture.

The size of the aperture is measured as an f-stop. Most commonly, the f-stop is a number in the geometric series 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11.0, 16.0, 22, 32. The series is designed so that the next higher stop cuts the light transmitted through the lens to half the value of the previous stop. An f-stop setting of 8.0 allows half as much light into the camera as a setting of 5.6. (The sequence is simpler than it looks. Each f-stop differs from its predecessor by the square root of two, the results rounded.) Table 23.1 lists the ISO standard (nominal) f-stops, the actual (computed) f-stops, and relative light values.

Although the sequence of numbers is now an arbitrary sequence, the value of the f-stop is scientifically defined. It is the focal length of the lens divided by the apparent aperture of the lens (not the actual size of the hole in the iris but the size of the hole visible through the lens—the glass in the lens can magnify the aperture or even make it appear smaller). For example, a lens with a 50-millimeter focal length set at f-stop 4 would have a visible aperture of 12.5 millimeters. This relationship leads to the common way of writing f-stop settings. A setting of four is usually written as f/4. In other words, it is the focal length divided by four.

Lenses are usually described by the widest aperture at which they can be set (for example f/1.4 or f/2.8). Sometimes the widest aperture setting falls between the numbers of the standard f-stop sequence (for example f/2.3), but the value represents the same concept.

Many zoom lenses are marked with two f-stops (for example f/2.8–4.3). The two values do not represent the total range of stops available from the lens. Rather, they represent the range of minimum f-stop values. The formula for determining the f-stop of a lens requires the size of the aperture to vary with the focal length of the lens at a constant f-stop setting. A longer lens requires a wider aperture for the same f-stop setting.

Zoom lenses are able to vary their focal lengths to change the size of the image they make in your camera. Nearly all zoom lenses automatically change the aperture as you zoom to maintain a constant f-stop setting. The physical diameter of the lens limits its maximum aperture—the hole in the iris can’t be bigger than the lens itself. When the focal length is set shorter, however, the largest possible aperture represents a wider (lower value) f-stop, which is desirable because it allows more light into the lens so that you can take photographs in dimmer light. Consequently, lens-makers let you take advantage of the wider f-stop settings at shorter focal lengths, and the minimum f-stop value varies with the focal length setting of the lens. The highest f-stop number represents the widest setting at the longest focal length setting of the lens. The lower value represents the widest f-stop setting possible at the most favorable focal length setting.

When using a lens, the aperture or f-stop setting can have a dramatic effect on the final image beyond setting exposure. The f-stop determines the depth of field (more correctly, the depth of focus) in the image.

Focal Length

Technically speaking, the focal length of a lens is the distance from its nodal point to the plane of focus of its image of an object an infinite distance from the lens (that is, where the image appears sharpest). Although this highly technical concept appears to have no practical value in judging cameras or lenses, focal length has an important ramification. It determines the field of view a lens provides or, the corollary, the size of the image. As a practical matter, a lens with a short focal length provides a wide field of view and makes things look smaller and farther away. A long lens provides a narrow field of view and large images that look closer. There are other aesthetic considerations we’ll discuss in the chapter about using a camera.

Wider, smaller, larger, and closer are all relative concepts. With lenses, such terms relate to a “normal” lens. For some reason not readily apparent, the photographic world has decided a normal lens has a field of view of 46 degrees. That is, the angle between the camera and the left side of what it sees is 46 degrees from a line drawn to the rightmost side of the image. On a 35 mm camera, that means a 50 mm lens is “normal.”

The heart of any digital camera is its image sensor, which actually captures the image, registering it in a form that can later be used. It detects photons and translates their energy into a minute electrical current that computer circuits can sense, amplify, digitize, and store.

Many people call image sensors CCDs, the name of the technology used by most digital camera image-sensing devices (charge coupled devices). Although CCDs are used in most cameras, some companies have adopted a similar technology that’s usually termed CMOS. Yes, it’s the same name as the memory used for system-configuration data in your computer, because the image sensors and the memory use the same semiconductor technology—Complimentary Metal-Oxide Semiconductor technology. Because of the use of garden-variety circuit technology, CMOS sensors tend to be less expensive, although they typically are not as sensitive as CCDs.

Although the term CCD usually is used in the singular, the actual image sensor of a digital camera requires an array of CCDs. The mere presence of light is not as important as its pattern, which makes up the actual image. A single CCD (or CMOS sensor) element registers only a single point. Consequently, image sensors are an array of individual CCD elements. In video cameras, the CCD elements get arranged as a matrix. Camera circuitry samples each element in turn to scan an image frame.

Size

Image sensors come in various sizes. Typically they measure one-quarter, one-third, or one-half inch (diagonally). All else equal, the larger the CCD, the greater the number of elements that can be packed into the array. The number of elements determines the resolution of the image signal produced by the CCD. This number is related to, but is not the same as, the number of pixels in an image.

The size of the sensor affects other elements of the camera design. For example, the coverage of the lens usually is tailored to match the sensor size. (That’s one reason why camera-makers cannot simply put CCDs inside ordinary 35 mm cameras—today’s practical sensors are substantially smaller than 35 mm film and would only register a fraction of the image from the lens.) Because most image sensors are smaller than the film used in ordinary cameras, interchangeable lenses often act “longer” on digital cameras. That is, a 50 mm lens from a standard film-based SLR might give the view of a 65 mm lens when twisted onto a digital camera. The sensor sees only the central part of the image from the lens. With many of today’s high-end digital cameras with interchangeable lenses, 35 mm lenses act as if their focal length were 1.3 times longer than on a film camera.

Resolution

The maximum possible resolution is set by the number of pixels that the CCD can sense. The keyword in digital photography currently is megapixel—that is, a million pixels. At one time photographers used the million-pixel mark as distinguishing good cameras from bad. Today the best consumer-model digital cameras have about five megapixels of resolution, which results in images comparable to 35 mm film.

The number of total pixels is directly related to the resolution of the image sensor. If you know the number of elements in an image sensor (video cameras are sometimes described by this figure), you can determine its highest possible resolution using these formulae:

Horizontal resolution = 4 * sqr(number of pixels/12)

Vertical resolution = 3 * sqr(number of pixels/12)

Unlike the measure of resolution of other computer peripherals, where each pixel requires an individual sensor for each color (three sensor elements equal one pixel), digital camera makers exaggerate their resolutions by using the total number of pixels for all colors to come up with a resolution figure. Most digital cameras use an array of sensors that alternate green with each of the other two colors. Using interpolation algorithms, the camera calculates three individual color values at each element position, regardless of its color sensitivity; in effect, creating three virtual pixels (one of each color) for each image sensor element. A megapixel camera thus has only one million sensing elements.

Sensitivity

Image sensors vary in their sensitivity to light. The best can detect a single photon. Those in digital cameras require dozens of photons to make a detected signal. It is similar to the rating of film speed. In fact, to give film photographers a better feel for their digital cameras, many digital cameras allow you to adjust their sensitivities to the equivalent of an ASA or ISO film speed rating (typically the ASA 100 or ASA 400 rating of the most popular films).

The inherent sensitivity of the image sensor is not, however, directly relevant to practical photography. Other aspects of the design of the camera overwhelm concerns about the native sensitivity of image sensors. The most important of these is the lens.

There are three kinds of camera that capture digital images. In theory, you can use any of the three to make digital images, although some work better for specific purposes.

Still Cameras

When anyone talks about digital cameras, they mean digital still cameras. That is, handheld cameras designed like the film cameras of old. You peer through a viewfinder, push a button, and take a picture.

The most important characteristic of digital still cameras is high resolution. The images they capture contain as much image information—detail—as the photographs taken by classic film cameras. The high-resolution images are suitable for printing as conventional photographs. Their quality is high enough you can even publish them in print magazines and books. But you can also take lower-resolution images—or reduce the resolution of high-resolution images—to make pictures for the Web and electronic publications. Depending on the memory you have available to your digital still camera, you can cram from a dozen to more than a hundred images into a single session.

To match their high resolution, digital still cameras have top-quality lenses characterized by their sharpness. Most digital still camera lenses also have optical zooming capabilities, ranging from 2x zooming in inexpensive cameras to a maximum (currently) of about 10x.

Digital still cameras are meant to work like their film-based counterparts—handheld or on a tripod—by using the available light or a flash. But most offer additional features. Some capture short video clips. Most have video outputs so you can plug them into your computer and capture moving images, just as you would with a video camera. You can also use this video output as a Web cam (providing your computer has a video input).

Video Cameras

Digital video cameras are made for capturing movies digitally. They are characterized by their fast capture rate. They have to snap at least 30 frames every second. But they do not need to have as much resolution as digital still cameras. Most have resolution that matches that of your computer’s most basic display mode, VGA. That means resolution of 640-by-480 pixels.

Because of the low resolution of digital video cameras, their lenses need not be as sharp as those of digital still cameras. On the other hand, to give you more versatility in movie-making, digital video cameras usually have extended zoom ranges, often higher than 30x.

Most digital video cameras also have single-shot modes in which they operate as digital still cameras—but with a difference. They produce video-like resolution rather than still-camera resolution when taking snapshots. You can also used a digital video camera for a Web camera if your computer has the proper input and software.

Web Cameras

Web cameras are the low end of the digital-photo spectrum. They have about the same resolution capabilities as a digital video camera (sometimes less) but skimp on the lens. Most have a fixed focus lens without zoom—which means low-cost, at least to manufacturers. Their low resolution level is tailored to match the needs of Webcasting and video messaging.

Most Web cameras have USB outputs inside of standard video outputs so you can plug them into computers that don’t have video inputs. With the right software, however, you can use a Web cam to make low-grade movies or still images. In fact, the quality you get from a Web cam may be enough for posting on the Web (what did you expect?) or even for putting images into electronic publications.

As with other input devices, scanners have their own control and signaling systems that must link to your software to be used effectively (or at all). Early scanners used their own proprietary application interfaces to relay commands and data. Consequently, each scanner required its own software or drivers. Oftentimes you could only use the scanner manufacturer’s own software to grab images.

Thanks to a concerted effort by the scanner industry, that situation has changed. Now you can expect any scanner to work with just about any graphics program. Moreover, scanning is consistent across applications. The same screens that control your scanner in PhotoShop appear in Corel Photo-Paint.

Central to this standardization is Twain. First released in early 1992, Twain is a scanner software interface standard developed by a consortium of scanner and software makers called (in its final form) the Working Group for Twain. The primary companies involved in forming the working group included Aldus Corporation, Caere Corporation, Eastman Kodak Company, Hewlett-Packard, and Logitech.

The Twain name requires some explanation. Twain is not an acronym, so only its initial letter needs to be capitalized. Rendering Twain in all capital letters is a typographic error. That said, the promoters of the standard usually write it in all capital letters as if it were an acronym.

Officially, the Twain developers have explained that the name is a reference to the purpose of the interface. Not an acronym, it derives from making the twain (an archaic word for two) meet, the two of the twain being applications and scanners. However, a few wags insist that Twain stands for “Technology Without An Interesting Name.”

When the Twain interface was being developed, it wore a number of different names. The most common of these were Direct-Connect and CLASP, the latter of which stands for the Connecting Link for Applications and Source Peripherals. The developers of Twain considered these and others as the formal names of the interface. After searching through lists of trademarks in use, however, they found so many conflicts they felt that lawsuits would be a distinct possibility if any of the developmental names were to be used. Instead they chose the name Twain to, in the words of one of the developers, “describe this interface which brings together two entities, applications and input devices.”

Twain links programs and scanner hardware, giving software writers a standard set of function calls by which to control the features of any scanner. One set of Twain drivers will handle any compatible scanning device. Because the Twain connection has two ends—your scanner and your software—both need to be Twain compatible for you to take advantage of the connection.

Twain defines its hardware interface as its Source. The Source is hardware or firmware in a scanner that controls the information that flows from the scanner into Twain. The scanner-maker designs the Source to match its particular hardware and interface. Your software links to the Twain Source through a Source Manager, which is essentially a set of program calls.

Twain takes the form of a software driver. The original driver was written in 16-bit code and takes the name TWAIN.DLL. The working group has also created a fully 32-bit version of the driver called TWAIN32.DLL. Both the 16- and 32-bit versions work with all versions of Windows since 95 (including 32-bit versions such as NT, 2000, and XP).

Beside their basic mechanism, scanners are distinguished by their features. Among these are whether the scanner can produce color images, its scanning speed, the dynamic range it can handle, its resolution, and whether it can recognize text characters and translate them into character rather image data. The availability of options such as transparency adapters, sheet feeders, and optical character recognition software also makes one model more suited to some applications than others.

Scanning Speed

Early color scanners give monochrome models a hefty edge in performance. The earliest color scanners were three-pass machines. That is, they required three passes to make a complete image, one pass for each of the primary colors. These ancient scanners used three separate light sources of different colors and took a full scan in each color. Nearly all modern scanners use one-pass designs. They have a single light source and rely on filtering in their photodetectors to sort out the colors. One-pass color scanners can operate just as quickly as monochrome models, although transferring a large color image measuring dozens of megabytes still takes longer than moving a monochrome image one-third its size.

The speed at which the scanning CCD moves across the image area is only one factor in the total time required to make a scan. Most scans require at least two separate passes. First, you command the scanner to make a pre-scan, which is a relatively quick, low-resolution pass across the image that helps you establish its brightness range and also lets you target a specific area for scanning. Then you make the actual scan at the resolution you want.

In addition, the interface used by a scanner influences the speed of scans, as noted later in the chapter. The high-resolution bit-images produced by the scanner represent a huge amount of data—megabytes—and a slow interface constricts this data’s flow.

If the scan you want to make is large, you also have to wait for image processing, both in the scanning software and in the host application. Very large scans can add minutes or more to the total scan time if you exceed the memory capabilities of your computer. Although Windows can take advantage of virtual memory to let you capture images of nearly any size, this technology uses your disk drive for extra storage space, which adds the seeking, writing, and reading times to the total time of your scan. If you plan to regularly make large scans, you’ll speed things up more by adding memory to your computer—in the dozens of megabytes—rather than looking for a faster scanner.

Dynamic Range

At one time, the base-level distinction between scanners was like that of television sets—color or monochrome. And, as with televisions, the monochrome variety is almost extinct. But color scanners aren’t all equally colorful. Some recognize more hues than others.

The compass of colors a scanner can discern is termed the scanner’s dynamic range. The most common means of expressing dynamic range is bit-depth, the number of bits needed to digitally encode the total color capacity. Most common scanners can distinguish 256 (8-bit), 1024 (10-bit), or 4096 (12-bit) brightness levels in each primary color. Just to make their scanners seem more capable, scanner manufacturers add up the total number of colors within the scanner repertory, so you’ll see 24-bit, 30-bit, and 36-bit color scanners.

The actual dynamic range and the bit-depth of a scanner are not necessarily the same. A high-quality scanner will be able to resolve the number of brightness levels its bit-depth implies. The bit-depth actually specifies the range of the analog-to-digital converters that convert the level detected by the scanner’s CCD sensors into digital signals. Taking advantage of that bit-depth requires that the scanned image be properly focused on the CCD sensor under optimal illumination. If the focus of the scanner’s optics is off, pixels will blur into one another, which lowers image contrast and the dynamic range of the scanner. Similarly, if the illumination provided for the image during the scan is uneven, the variations will wipe out some of the available brightness levels of the dynamic range. Consequently, two scanners with the same number of bits quoted for their dynamic range may, in fact, have different actual dynamic ranges.

Most computers can, of course, display from 256 to 16.7 million different hues (that is, 8-bit to 24-bit color). When that is more than you or your software wants to manage, the scanner’s palette can easily be scaled back, either through hardware controls or through software, to an even smaller bit-depth. With even a minimal 24-bit scanner capable of giving your software more than enough color, the extra bits of higher-cost scanners might seem superfluous.

Those extra bits are very useful, however, when the scanner preprocesses image data before passing it along to your computer. A scanner with 36-bit dynamic range can capture all the shades and hues of an image and let you process them down into 24-bit color for your computer to use. You get to choose how to handle the conversion, compressing the dynamic range or cutting off colors you don’t want to use. The extra bits ensure that your scanner can capture all the detail in the darkest shadows and brightest highlights. When you use a transparency adapter, greater dynamic range helps you compensate for thinner or denser originals, potentially yielding workable scans from transparencies you can barely see through.

The trend among high-end scanners is toward a dynamic range of 48 bits, giving you a greater range of manipulation. Note, however, that most existing software for image manipulation cannot yet handle 48-bit images (although the current release of Adobe Photoshop can).

Many scanners have automatic modes through which they determine the proper brightness and contrast ratios to take best advantage of the translation of the scanner’s dynamic range into the 24-bit level of color (or other level of color) used by your computer. The most common means of making this optimization is to pre-scan the image. The scanner then checks the brightest and darkest points of the scanned area. Using these values to establish the actual range of brightness and color in the image, the scanner can adjust its transformation to yield the image with the greatest tonal range to your applications.

D-max

With slide scanners, another factor influences your ability to work with marginal slides and negatives—the maximum image density, usually abbreviated as D-max, that the scanner can handle. A more dense image is darker—there’s more dye or silver in the image. Because slide scanners work by shooting light through the slide or negative, a very dense image may prevent any light from getting through at all. The D-max indicates how dense an image can be before the scanner can no longer distinguish the light shining through it. Any part of an image that’s more dense than the scanner’s D-max rating blocks up as solid black (or, if you’re scanning a negative, solid white).

Technically speaking, the density of a photographic negative or slide is the ratio of the intensity of light shining through the image over the intensity of light that actually gets through, expressed as a logarithm. The scientific formula for density is as follows:

Density = log (incident light/transmitted light)

As a practical matter, the best of today’s slide scanners cope with a D-max of about 4.2. That means they can detect light that’s diminished by a factor of more than 10,000. A scanner with a D-max of less than 3 will have difficulty dealing with the full range of image brightnesses, even on properly exposed film.

Resolution

Scanners differ in the resolution at which they can capture images. All scanners have a maximum mechanical limit on their resolution. It’s equal to the smallest step that their sensor can be advanced; typically a minimal scanner will start with about 300 dots per inch and go up from there in regular steps such as 600, 1200, then 2400 dots per inch. Special-purpose slide scanners achieve resolutions as high as 10,000 dots per inch. Because it represents the limit of the quality the scanner hardware is able to resolve, this measurement is often termed the hardware resolution of the scanner. Another term for the same value is optical resolution.

Beyond the mechanical resolution of a given scanner, the control software accompanying the scanner often pushes the claimed resolution even higher, to 4800 or even 9600 dots per inch, even for an inexpensive scanner. To achieve these higher-resolution figures, the control software interpolates dots. That is, the software computes additional dots in between those that are actually scanned.

This artificial enhancement results in a higher resolution value quoted for some printers as interpolated resolution. Although interpolating higher resolution adds no more information to a scan—which means it cannot add to the detail—it can make the scan look more pleasing. The greater number of dots reduces the jaggedness or stair-stepping in the scan and makes lines look smoother.

The new dots created by interpolation add to the size of the resulting scanned file, possibly making a large file cumbersome indeed. In that interpolation adds no new information, it need not be done at the time of scanning. You can store a file made at the mechanical resolution limit of your scanner, then later increase its apparent resolution through interpolation without wasting disk space storing imaginary dots.

As with colors and shades of gray, a scanner can easily be programmed to produce resolution lower than its maximum. Lower resolution is useful to minimize file size, to match your output device, or simply to make the scanned image fit on a single screen for convenient viewing. Although early scanners and their control software shifted their resolution in distinct increments—75, 150, and 300 dpi, for example—modern scanner-plus-software combinations make resolution continuously variable within wide limits.

The actual hardware resolution of a scanner is fixed across the width of the image by the number of elements in the CCD sensor that determines the brightness of each pixel. The hardware resolution along the length of the scan is determined by the number of steps the CCD sensor takes as it traverses the image area. The size of these steps is also usually fixed. Scanning software determines lower as well as higher resolution values by interpolating from the hardware scan. Consequently, even when set for 50 dpi, a scanner will sense at its hardware resolution level, deriving the lower-resolution figure through software from its higher capabilities.

Transparency Adapters

As with people, scanners are not blessed with the ability to see in the dark. To make a proper scan—that is, one that doesn’t resemble a solar eclipse in a coal mine—the scanner needs a light source. All scanners have their own built-in and usually calibrated light sources. In drum and flatbed scanners, the light sources are inside the mechanism, typically one or three cold cathode tubes that glow brightly. Handheld scanners often use light emitting diodes (LEDs) as their illumination source. In any case, in normal operation the light reflects from the material being scanned, and the CCD sensors in the scanner measure the brightness of the reflected light.

Some source materials fail to reveal their full splendor under reflected light. The most important of these are transparencies such as photographic slides or presentation foils. These are designed to have light shine through them (that is, transmitted light).

To properly scan these media, the scanner must put the media between its light source and its sensor. Slide scanners have the source for transmitted light built in. Most other desktop scanners have an optional secondary source for transmitted light called a transparency adapter. The secondary light source tracks the CCD sensor as it scans across the image, but from the opposite side of the original.

Most commonly the transparency adapter takes the form of a thicker cover over the glass stage on which you lay your originals to be scanned. A few scanners have add-on arms that scan over the top of the transparencies you lay on the stage. The latter style works well but does not hold original transparencies as flat against the stage as do the former.

Optical Character Recognition

Scanners don’t care what you point them at. They will capture anything with adequate contrast, drawing or text. However, text captured by a scanner will be in bit-image form, which makes it useless to word processors, which use ASCII code. You can translate text in graphic form into ASCII codes in two ways—by typing everything into your word processor or by Optical Character Recognition (OCR). Add character-recognition software to your scanner, and you can quickly convert almost anything you can read on your screen into word processor, database, or spreadsheet files. Once the realm of mainframe computers and special hardware costing tens of thousands of dollars, OCR is now within the reach of most computers and budgets.

Early OCR software used a technique called matrix matching. The computer would compare small parts of each bit-image it scanned to bit-patterns it had stored in a library to find what character was the most similar to the bit-pattern scanned. For example, a letter A would be recognized as a pointed tower 40 bits high with a 20-bit wide crossbar.

Matrix matching suffers a severe handicap—it must be tuned to the particular typeface and type size you scan. For example, an italic letter A has a completely different pattern signature from a roman letter A, even within the same size and type family. Consequently, a matrix-matching OCR system must have either an enormous library of bit-patterns (requiring a time-consuming search for each match) or the system must be limited to matching a few typestyles and fonts. Even then, you will probably have to tell the character-recognition system what typeface you want to read so it can select the correct pattern library. Worse, most matrix-matching systems depend on regular spacing between characters to determine the size and shape of the character matrix, so these systems work only with monospaced printing, such as that generated by a typewriter.

Most of today’s OCR systems use feature matching. Feature-matching systems don’t just look and compare; they also analyze each bit-pattern that’s scanned. When it sees the letter A, it derives the essential features of the character from the pattern of bits—an up-slope, a peak, and a down-slope with a horizontal bar across. In that every letter A has the same characteristic features—if they didn’t your eyes couldn’t recognize each one as an “A,” either—the feature matching system doesn’t need an elaborate library of bit-patterns to match nearly any font and type size. In fact, feature-matching recognition software doesn’t need to know the size or font of the characters it is to recognize beforehand. Even typeset text with variable character spacing is no problem. Feature-matching software can thus race through a scan very quickly while making few errors.

Sheet Feeders

The typical OCR application involves transferring the information content of multiple pages into electronic form. You must, of course, scan each page separately to derive its information content. With long documents, the chore is time consuming and usually not the most productive way to spend your working hours.

A sheet feeder automatically runs each sheet of a multiple-page document through a scanner. Although a sheet feeder is easiest to implement with a drum scanner, because the scanner has to put the paper in motion anyway, some flatbed scanners have built-in or optional sheet feeders as well.

Sheet feeders are useful primarily for OCR applications. Graphic scanning usually involves individual setup of each page or image. Actually loading a page into the scanner is a trivial part of the graphic scan. Adding a sheet feeder to a scanner used primarily for graphics is consequently not cost effective.

Sheet feeders require loose sheets. They cannot riffle through the pages of a book or other bound document. When a job requires high productivity and the information is more valuable than the printed original, some people cut apart books and similar materials for scanning using a sheet feeder. In any case, you’ll probably find a staple-puller to be a worthy accessory to your sheet feeder.

Electrical Interfacing

At least six different interfaces designs are or have been used by scanners: Small Computer System Interface (SCSI), General-Purpose Interface Bus (GPIB), standard serial, parallel, USB, and proprietary. Almost all current products rely on parallel, SCSI, or USB connections.

Parallel models plug into legacy printer ports. Most have special cables or connectors that allow you to link your printer to the same port used by the scanner. With modern port/driver software, parallel-interfaced scanners are the easiest to get running—they come to life almost as soon as you plug them in (and install the drivers). The parallel interface is also inexpensive.

The downside of the parallel connection is performance. It is the slowest of the scanner links, and it may double the scan time of a typical page.

The SCSI interface, on the other hand, is fast—the fastest scanner connection in use. The penalty is, of course, the need to tangle with a SCSI connection. This need not be a problem. When the SCSI-based scanner is the only device plugged into a SCSI port, getting the scanner to work is about as easy as with a parallel port. Adding a scanner to a long SCSI chain is as fraught with problems as linking any additional SCSI device.

The other problem with SCSI-based scanners is that they require a SCSI port. Most SCSI-based scanners come with their own SCSI host adapters and cables. (This is one reason the SCSI interface adds to the cost of a scanner.) Installing the adapter in an expansion slot complicates the installation process. Worse, if you want to use a SCSI scanner with a notebook computer, you’ll need to purchase a PC Card SCSI adapter.

USB scanners fit in the middle. Although they require a free USB port, nearly all new computers (including notebooks) have at least one. Most have at least two. Scanners fit readily into USB’s Plug-and-Play system, but they suffer from the same teething difficulties as other USB products. Although USB scanning is quicker than parallel, it is not as faster as SCSI scanning.

How the view of the scanning sensor moves to that following line is the fundamental design difference between scanners. Somehow the long line of sensing elements must shift their attention with extreme precision over the entire surface of the image to be captured. Nearly all scanners require a mechanical sweep of the sensors across the image, although a few low-resolution scanners use video technology to sweep their view electronically.

To make a sweep in a mechanical scanner, engineers have devised two primary strategies. One requires the image sensor to move across a fixed original, like you examining a statue in a museum by walking around it. The other moves the original in front of a fixed scanner the same way you might examine an apple for intruders that have bored inside, by holding it in your hand and turning it around in front of your eyes. With a video scanner, nothing moves except an electron beam.

Drum Scanners

The very first scanners helped newspapers and wire services send images across the country with the ease of telegraphing messages. Someone in one newspaper office wrapped a photo around a metal cylinder or drum, and the scanner spun the drum around while a single light sensor checked the brightness of the photo at a single spot—which became a chain of observations as the drum continually spun the image under the watchful photo-eye. With every spin of the drum, the light detector moved slightly down the photo until it got to see the entire image. A matching spinning drum covered with light-sensitive paper (like photographic film) at the other end of the connection created the image by scanning it with a light beam.

Engineers adapted this same moving-cylinder approach into the drum scanner. Instead of a single photo-eye, the drum scanner uses a linear array so that a single spin of the drum covers the entire image.

Operationally, the drum scanner works like a printing press in reverse. You feed a piece of paper that bears the image you want to capture into the scanner, and the paper wraps around a rotating drum that spins the image past a sensor string that’s fixed in place inside the machine.

The drum design lends itself to document processing. The mechanism puts the paper being scanned in motion, so adding a page feeder is a relatively simple addition. Because of their orientation to scanning printed pages, drum scanners are sometimes termed page scanners.

Today, most consumer-model drum scanners lack the drum that gave them their name. Instead of wrapping each sheet of paper around a drum, most scan each sheet as it slides through their mechanisms flat. The image sensor peers through a narrow slit across the paper path, recording line after line as the paper rushes through. Expensive, precision scanners used in the graphic arts industry still cling to the classic drum design because of its precision and simplicity.

The drum scanner mechanism imposes a stiff penalty—it allows only thin, flexible images to be scanned. In general, a drum scanner accepts only sheets of normal paper. Books (at least while intact) and solid objects are off limits. Moreover, most drum scanners accept only a few sizes of paper, typically the 8.5-by-11-inch sheets of business documents. Consequently, drum-scanning technology today is restricted to high-volume (and expensive) document processing systems used in big businesses.

Flatbed Scanners

The flatbed scanner takes the opposite tack. Instead of moving the paper to scan it, the flatbed scanner moves its line-up of sensors down the sheet. It earns its “flatbed” name from the flat glass surface, the bed upon which you must place the item to be scanned, face down. In most flatbeds, the linear array of scanning sensors is mounted on a bar that moves under the glass, automatically sweeping across the image. The clear glass lets the sensors see up to the image. In addition, the glass protects the sensors and gives them a target fixed in place at a preset distance from the scanner, which keeps things in focus.

Flatbed scanners have precision mechanisms that step the sensors or image a small increment at a time, each increment representing a single scan line. The movement of the mechanism, which is carefully controlled by the electronics of the scanner, determines the width of each line (and thus the resolution of the scanner in that direction).

Flatbed scanners are like copying machines in that anything you can lay flat on their glass faces can be scanned—books, magazines, sections of poster, even posteriors and other parts of your anatomy if you get imaginative, bored, or drunk. Of course, the scanned image can be no larger than the scanner bed.

In the past, the chief drawback of the flatbed scanner has been price. But manufacturers have refined flatbed technology to the point flatbed scanners are sometimes given away free to entice you to buy a particular computer. Although top-quality flatbed scanners for graphic arts professionals still demand hefty prices, you can buy an entirely satisfactory flatbed scanner for little more than $50. Those prices have made scanners that use other technologies scarce.

Hand Scanners

Hand scanners are a variation on the flatbed design (believe it or not!) that make you the motive force that propels the sensor over the image. You hold the T-shape hand scanner in the palm of your hand and drag it across the image you want to scan. A string of sensors peers through a plastic window in the bottom of the hand scanner to register the image.

Hand scanners must cope with the vagaries of the sweep of your all-too-human hand. If you move your hand at a speed other than that at which the scanner expects, lines will be scanned as too wide or too narrow, resulting in image distortion—at best the aspect ratio may be off, at worse the scanned image will look as wavy as the Atlantic under the influence of an errant typhoon. To avoid such disasters, the hand scanner uses a feedback mechanism that tracks the position of the image. Most have a roller that presses down against the image you’re scanning to sense how fast you drag the scanner along. The rate at which the roller spins gives the scanner’s electronics the feedback it needs about scanning speed. From this information, the software that controls the hand scanner can give each scanned dot its proper place.

At one time, hand scanners were a low-cost alternative to flatbed designs. Because they omitted the most expensive parts of most scanners—the precision mechanism for moving the paper or sensor—they had an automatic edge in price. With the plummet in prices of flatbed scanners, however, hand scanners were hard-pressed to keep up. A low-cost flatbed is now likely to be less expensive than a hand scanner. Consequently, few hand scanners are left on the market.

Those remaining have survived because of the chief remaining advantage of the hand scanner—portability. Hand scanners are compact and easy to carry. You could plug one into your notebook computer and carry the complete system to the neighborhood library to scan from books in its collection.

In addition, using a hand scanner can be quicker than using a flatbed because you can make fast sweeps of small images instead of waiting for the lumbering mechanism of a flatbed to cover a whole sheet. Hand scanners may also adapt to some nonflat surfaces and three-dimensional objects. For example, most will easily cope with the pages of an open atlas or gothic novel—although few can do a good job on a globe or watermelon.

On the downside, the small size of the hand scanner means a single pass of the scanner will cover an image no more than about four inches wide. Although that’s enough for a column of text (and most scanners offer a means of pasting together parallel scans of larger drawings and photos), the narrow strips of scan make dealing with large images inconvenient. On the other hand (and in the other direction), because a hand scanner is not limited by a scanning mechanism, it can allow you to make absurdly long scans, typically limited only by the scanning software you use.

Note that hand-scanning is like typing—it’s a learned skill. To use a hand scanner effectively, you’ll have to practice until you learn to move the scanner smoothly and at the proper speed, which means very slowly at high resolutions.

Video Scanners

A video scanner is the electronic equivalent of a photographic copy stand. That is, the scanner operates like a camera, taking in a view of the entire image in a single look. That makes a video scanner fast—capturing an image takes a fraction of a second. You’ll spend more time setting up the image or object to be scanned than the scanner needs to scan.

Typically a video scanner uses a conventional video camera to capture an image. Most video scanners permanently mount the camera on a stand and give you a stage on which you put the item to be scanned. The stage may have a backlight to allow you to scan photographic slides or negatives, or it may be a large bed for sheets of paper or even three-dimensional objects.

Video scanners avoid all the problems and inaccuracies imposed by mechanical scans. They have the potential for the greatest precision. Typically, however, video scanners yield the lowest quality. Like a video camera, video scanners require a CCD element for every pixel they scan, and affordable two-dimensional CCD arrays have only a few hundred thousand pixels. Because video scanners use the same CCD arrays as video cameras, they have the same resolution as video cameras, not measured in dot per inch but in pixels across the entire image. They are suited to snapshots and catalog illustration but not high-quality scans.

Photo Scanners

As their name implies, photo scanners are special-purpose devices aimed at capturing digital images from photographic prints. Most use an adaptation of drum-scanner technology. They move the original rather than the sensor, sliding the photo past the image sensor, flat.

Photo scanners can be quicker to use because you only need to slide your snapshots in like you’re feeding dollar bills into a vending machine. Because their mechanisms are inherently less complex than those of flatbed scanners, at one time dedicated photo scanners had a price edge. With the current generation of low-cost flatbed scanners, however, that advantage has vanished. A flatbed can do everything a photo scanner can—and more—but if all you need to scan is photos, you’ll have a quicker and easier time with the dedicated device.

Slide Scanners

The slide scanner is not a special technology but rather a special implementation of flatbed or video scanner technology. A slide scanner is a transmissive scanner rather than reflective. That is, it registers the light that is transmitted through an image rather than the light reflected from the image. The source of illumination is on one side of the image, and the image sensor is on the other. The image must be on a transparent medium.

A flatbed-style slide scanner is optimized for the higher-resolution needs of scanning small photographic transparencies or negatives but relies on a modified flatbed scanner mechanism. It needs only more precision in its control because of the smaller size of the scanned area of negatives and slides (and the correspondingly high resolution required). A video slide scanner is subject to the same limitations as any video scanner—chiefly, low resolution—but gives you an inexpensive means of capturing limited-resolution snapshot-quality images from slides and negatives.

Mice convert the motions they detect into a digital code that can be processed or analyzed by your computer. The only loose end is what code the mouse uses. A standard mouse code would help software writers craft their products to better take advantage of mice. A standard mouse code would be so useful, in fact, that the industry has come up with four distinct standards, called mouse protocols, all of which were in use at one time. These standards were developed by four of the major forces in the mouse industry, and each bore its originator’s name. These include Microsoft, Mouse Systems Corporation (for a period known as MSC Corporation), Logitech, and IBM Corporation. The first three were designed for individual mouse products created by the respective companies. The IBM protocol was introduced with the PS/2 series of computers, which came equipped with a built-in jack that accepted a mouse. Other pointing devices use their own protocols, complicating matters further.

In truth, you don’t need to know the details of any of these protocols. That’s the challenge handled by the software driver used by your pointing device. And that’s why you must match the right driver software to your pointing device.

Today, the Microsoft mouse protocol is the most prevalent. Most generic mice will work as a Microsoft mouse if you don’t have a specific driver for your mouse. But the generic Microsoft mouse protocol does not take advantage of the specific features of the products of other manufacturers.

In normal circumstances, Windows will find and recognize your mouse or other pointing device and install the proper driver software for it. Because Windows has a large repertory of built-in drivers, you may not need a specific driver for your device. You can also install a new mouse manually. Windows gives you at least two ways of accomplishing the installation: through the Add New Hardware icon in Control Panel or the Mouse icon in Control Panel (using the General tab and clicking the Change button).

To communicate its codes to your computer, any pointing device needs to be connected in some way. Notebook computers have it easy—signals can take the direct route because they are built in. Desktop systems require some kind of port to which you can connect your mouse or other pointing device. Most systems today have a dedicated mouse port. But the industry-wide push to eliminate legacy ports of any kind, as well as the need for alternative pointing devices, are making the USB interface popular (at least among computer-makers). A few mice using the legacy RS-232C serial port are also available.

Mouse Port

The most popular way to connect a mouse to a desktop computer is via the mouse port. Physically the mouse port resembles a keyboard jack. Both use a six-pin miniature DIN connector. The design owes its heritage to the first IBM Personal Systems/2 (PS/2) and consequently is sometimes termed a PS/2 mouse port. The only lasting change affected by the computer industry on this port design has been color-coding. To distinguish a mouse port from a keyboard port, the mouse port jack and connector are colored aqua, a bluish green. (Keyboard ports are purple.)

Electrically the mouse port is also similar to a keyboard port, using low-speed serial technology to send signals from mouse to computer using its own interrupt to avoid conflicts with other devices. This design limits the use of the mouse port because it is not fast enough to handle the needs of more sophisticated pointing devices.

USB Port

True to the “universal” in its name, the USB port accommodates mice and any other kind of pointing device. The port, even in its early Version 1.0 form, offers enough speed for the most elaborate pointing devices. Moreover, because pointing devices are not used during bootup before the operating system and its drivers load, there’s no need for special BIOS provisions for handling USB mouse control. The hardware installation of a USB-based pointing device requires only plugging it in, which is why this connection system is preferred.

Serial Mice

The serial port was the first connection system used by mice, and its shortcomings provided the incentive to develop the dedicated mouse port. In general, mice make no onerous demands on the serial port. They operate at a low communication rate (1200 bits per second) and adapt to any available port thanks to driver software. But putting a mouse on a serial port is hardly trouble-free. Because every mouse movement generates a serial-port interrupt, if your system has more than two serial ports, you can easily generate interrupt conflicts with a mouse. Because serial ports 1 and 4 (that is, the ports that DOS calls COM1 and COM4) share interrupt 4, and serial ports 2 and 3 (COM2 and COM3) share interrupt 3, a mouse can conflict with another device connected to the other port sharing its interrupt. If you have a serial mouse, it’s always best to plug your mouse into the port that does not have to share its interrupt (for example, COM1 if your computer has three serial ports) to avoid surprises—the kind that can crash your computer. Better still, use a mouse port or USB mouse.

Wireless

The most irksome trait of the mouse is the tail that inspired its name. The cord dangling from the far end of the mouse is a desktop hazard, liable to snag whatever you’re working on. Thank goodness that inkwells are about as likely as chimera on modern desks; otherwise, you could count on your mouse making your work a uniform ink-stained blue at least once a day. Mice consequently benefit from wireless technology even more than keyboards.

Making a mouse wireless is much the same as cutting the keyboard cord. The wireless link operates between the mouse and a base station, which connects to your computer through one of the aforementioned standard mouse interfaces. The wireless link is proprietary, allowing the manufacturer to chose whatever technology and design best fits the purpose and price of the product.

Infrared (IR) technology, although chosen by some manufacturers, does not mate with the typical cluttered desktop environment. The IR sensor in the base station must be able to see the mouse—literally—and any object in the way can create errors of omission, undetected mouse movements. If you confine your mouse to a pad, you can put the base station adjacent to the pad without risk of losing communication. But with today’s optical mouse technologies, the temptation is to move your mouse on any suitable desktop bare spot, which may not be near the base station.

Radio technology is a better match for the lifestyle of the modern mouse, and most manufacturers have shifted to it. Because radio waves penetrate most desktop objects, you can put the base station almost anywhere and still expect your mouse to work.

If you’re thinking of going wireless, a better choice is to buy a wireless keyboard and mouse as a package so you need only one base station for both.

When you drive your car, you don’t use a keyboard and mouse—at least not yet. When you’re racing along Santa Monica Boulevard in a round of Grand Theft Auto 3, you probably don’t want to type in commands to mash the accelerator through the floor and hang a flying Louie. You want the experience and the control system to be as real as possible without live ammunition. You want a real steering wheel, real pedals, and the real aroma of searing rubber and SAE30 motor oil flaming out your exhaust pipe.

So far electronic devices aren’t very adept at generating odors (although bad software really does stink), but some special hardware can help make you more adept in playing games—mechanisms that give you better control. Computer people have a clever name for the devices that help you control games. They call them game controllers.

The history of game controllers goes back to the first personal computers and beyond. As soon as computers graduated from the workbench to the television set, people played games on them. For control, they used joysticks. Even the business-oriented IBM Personal Computer made provisions for joysticks and a two-player option called game paddles using a special interface called the game port. But such primitive control systems are as passé as Pong, the first real video game.

Today’s elaborate wheel-and-pedal combinations are possible thanks to the architectural changes made by Windows. The game port hardware interface defined the control afforded by joysticks—two position sensors and a single pushbutton. Thanks to an additional layer of interface abstraction (driver software) and a better connection system (USB), designers are free to create any kind of control system they want when working with Windows.

The essential part of any game controller is one or more sensors or transducers. A sensor samples a physical measurement—be it brightness, distance, direction, or weight. A transducer converts or translates a physical property or change into an electrical signal. (The line between them is hard to draw—transducers are sensors, but not all sensors are transducers.) A pressure sensor, for example, is a transducer that varies its output with how hard you press on it (say, by hammering the accelerator). The magnetic or optic motion sensors in mice are not generally considered transducers.

The difference isn’t what matters. Rather, the game controller depends on being able to sense what you do—push a pedal, spin a wheel, pull back on a throttle or stick—and quantify it. The controller then takes the result of that quantification, translates it into a digital code, packages the information in a data packet, and whisks it out to your computer through its interface. The driver software in your computer receives the packet from the interface and translates its content into a form compatible with your game (or other program) and passes it along through the program’s Application Program Interface (API). Your game then decides what to do with the control data.

There’s no limit to the number of controls possible using this system. You could have a game controller as complex as the bridge of the Starship Enterprise. (Actually, that’s easy because the bridge is just a set, and none of the controls really work.) The controller designer is free to create anything imaginable (but technically and economically feasible, of course). Moreover, the system works both ways, so designers can put feedback mechanisms—for example, controllers that shake you up so you know when you’re really applying power—to make games more realistic. The only essential element is that the game software understand the information sent by the controller. Controller designers must match the requirements of game APIs. The software driver and interface are simply the glue that holds everything together.

Invented by IBM, which has trademarked the name and patented the technology, the TrackPoint system was developed by Ted Selker and Joseph D. Rutledge of IBM’s Thomas J. Watson Research Center and first used in IBM notebook computers. In principle, the pointing stick is a miniature joystick that’s stuck between the G and H keys of a conventional keyboard. The pointing stick protrudes just two millimeters above the normal typing surface. Its position enables you to maneuver it with either index finger while the rest of your fingers remain in the home row. Because in normal touch-typing your fingers should never cross the G/H boundary, it does not interfere with normal typing. The selection function of mouse buttons is given over to bar keys at the lower edge of the keyboard, adjacent to the spacebar. Figure 21.2 shows the placement of the TrackPoint.

The nub—which typically is removable in case you wear it smooth—mechanically connects to a pair of solid-state pressure sensors mounted at right angles to one another. When you press against one side of the TrackPoint device, it senses the pressure you apply even though the nub itself does not move. The TrackPoint electronics and software driver convert this pressure data into an indication of relative motion. The harder you press, the greater the signal the pressure sensor generates and the faster it tells your computer to move the mouse pointer. The paired sensors give you two axes of control corresponding to moving the mouse along its X and Y axes.

The TrackPoint system has several advantages in addition to its favorable typing location. It is entirely solid state and sealed so it, like a TouchPad, is environmentally rugged. It has no moving parts to wear out, except for the pointing nub, which you can readily replace. The disadvantage is that many people find it unnatural to use until they have acquired experience using it.