In principal, a digital camera is similar to a traditional film-based camera. There's a viewfinder to aim it, a lens to focus the image onto a light-sensitive device, some means by which several images can be stored and removed for later use, and the whole lot is fitted into a box. In a conventional camera, light-sensitive film captures images and is used to store them after chemical development. Digital photography uses a combination of advanced image sensor technology and memory storage, which allows images to be captured in a digital format that is available instantly - with no need for a "development" process.

Although the principle may be the same as a film camera, the inner workings of a digital camera are quite different, the imaging being performed either by a charge coupled device (CCD) or CMOS (complementary metal-oxide semiconductor) sensors. Each sensor element converts light into a voltage proportional to the brightness which is passed into an analog-to-digital converter (ADC) which translates the fluctuations of the CCD into discrete binary code. The digital output of the ADC is sent to a digital signal processor (DSP) which adjusts contrast and detail, and compresses the image before sending it to the storage medium (flash memory or disc). The brighter the light, the higher the voltage and the brighter the resulting computer pixel. The more elements, the higher the resolution, and the greater the detail that can be captured.

The CCD or CMOS sensors are fixed in place and it can go on taking photos for the lifetime of the camera. There's no need to wind film between two spools either, which helps minimize the number of moving parts.

CCD

The CCD is the technology at the heart of most digital cameras, and replaces both the shutter and film found in conventional cameras. It's origins lie in 1960s, when the hunt was on for inexpensive, mass-producible memory solutions. It's eventual application as an image-capture device hadn't even occurred to the scientists working with the technology initially.

At Bell Labs in 1969, Willard Boyle and George Smith came up with the CCD as a way to store data. The first imaging CCD, with a format of 100x100 pixels, was created in 1974 by Fairchild Electronics. By the following year the device was being used in TV cameras for commercial broadcasts and soon became commonplace in telescopes and medical imaging systems. It was some time later before the CCD became part of the high-street technology that is now the digital camera.

How a CCD works - It works like an electronic version of a human eye. Each CCD consists of millions of cells known as photosites or photodiodes. These are essentially light-collecting wells that convert optical information into an electric charge. When light particles known as photons enter the silicon body of the photosite, they provide enough energy for negatively-charged electrons to be released. The more light that enters the photosite, the more free electrons are available. Each photosite has an electrical contact attached to it, and when a voltage is applied to this the silicon below each photosite becomes receptive to the freed electrons and acts as a container for them. Thus, each photosite has a particular charge associated with it - the greater the charge, the brighter the intensity of the associated pixel.

The next stage in the process passes this current to what's known as a read-out register. As the charges enter and then exit the read-out register they're deleted and, since the charge in each row is "coupled" to the next, this has the effect of dragging the next in behind it. The signals are then passed - as free of signal noise as possible - to an amplifier and thence on to the ADC.

The photosites on a CCD actually respond only to light - not to color. Color is added to the image by means of red, green and blue filters placed over each pixel. As the CCD mimics the human eye, the ration of green filters to that of red and blue is two to one. This is because the human eye is most sensitive to yellow-green light. As a pixel can only represent one color, the true color is made by averaging the light intensity of the pixels around it - a process known as color interpolation.

Recognizing a glass ceiling in the conventional charge-coupled device (CCD) design, Fujifilm has developed a new, radically different CCD with larger, octagonal-shaped photosites situated on 45-degree angles in place of the standard square shape. This new arrangement is aimed at avoiding the signal noise that has previously placed limits on the densities of photosites on a CCD, providing improved color reproduction, a wider dynamic range and increased sensitivity, all attributes that result in sharper, more colorful digital images.

CMOS

1998 saw CMOS sensors emerge as an alternative image capture technology to CCDs. The CMOS manufacturing processes are the same as those used to produce millions of processors and memory chips worldwide. As these are established high-yield techniques with an existing infrastructure already in place, CMOS chips are significantly less expensive to fabricate than specialist CCDs. Another advantage is that they have significantly lower power requirements than CCDs. Furthermore, whilst CCDs have the single function of registering where light falls on each of the hundreds of thousands of sampling points, CMOS can be loaded with a host of other tasks - such as analogue-to-digital conversion, load signal processing, handling white balance and camera controls, and more. It's also possible to increase CMOS density and bit depth without bumping up the cost.

For these and other reasons, many industry analysts believe that eventually, almost all entry-level digital cameras will be CMOS-based and that only midrange and high-end units will use CCDs. At the start of the new millennium, CMOS technology clearly had a way to go before reaching parity with CCD technology.

However, it's prospects were given a major boost in late 2000 when Silicon Valley-based Foveon Inc. announced it's revolutionary X3 technology and the manufacture of a CMOS image sensor with about 3 times the resolution of any previously announced photographic CMOS image sensor and more than 50 times the resolution of the most commonly manufactured CMOS image sensors low-end consumer digital cameras of the time.

Prior to this, CMOS image sensors had been manufactured using 0.35 or 0.50 micron process technology, and it had been generally accepted that 0.25 represented the next round of product offerings. Foveon's 16.8 million pixel (4096x4096) sensor is the first image sensor of any size to be manufactured with 0.18 micron process technology - a proprietary analog CMOS fabrication process developed in collaboration with National Semiconductor Corporation - and represents a two generation leap ahead of the CMOS imager industry. The use of 0.18 micron processing enables more pixels to be packed into a given physical area, resulting in a higher resolution sensor. Transistors made with the 0.18 micron process are smaller and therefore do not take up as much of the sensor space, which can be used instead for light detection. This space efficiency enables sensor designs that have smarter pixels which can provide new capabilities during the exposure without sacrificing light sensitivity.

Comprising nearly 70 million transistors, the 4096x4096 sensor measures 22mm x 22mm and has an estimated ISO speed of 100 with a dynamic range of 10 stops. In the 18 months following its release the sensor is expected to be seen in products for the high-quality professional markets - including professional cameras, film scanners, medical imaging, document scanning and museum archiving. In the longer term, it is anticipated that the sensor's underlying technology will migrate down to the larger, consumer markets.

Picture quality

The picture quality of a digital camera depends on several factors, including the optical quality of the lens and image-capture chip, compression algorithms, and other components. However, the most important determinant of image quality is the resolution of the CCD. The more elements, the higher the resolution, and thus the greater the detail that can be captured.

In 1997 the typical native resolution of consumer digital cameras was 640x480 pixels. A year later as manufacturing techniques improved and technology progressed the emergence of "megapixel" cameras meant that the same money could buy a 1024x768 or even a 1280x960 model. By early 1999, resolutions were as high as 1536x1024 and before the middle of that year the two megapixel barrier had been breached, with the arrival of 2.3 million CCDs supporting resolutions of 1800x1200. A year later the unrelenting march of the megapixels saw the three megapixel barrier breached, with the advent of 3.34 megapixel CCDs capable of delivering a maximum image size of 2048x1536 pixels. The first consumer model 4 megapixel camera appeared in mid-2001, boasting a maximum image size of 2240x1680 pixels. We now see 6, 8 and 10 megapixel digital cameras in the consumer market.

At this level, raw resolution is arguably little more than a numbers game and secondary to a digital camera's other quality factors. One of these - and almost as important to the quality of the final image as the amount of information the CCD is capable of capturing in the first place - is how cleanly the information is passed to the ADC.

The quality of a CCD's color management process is another important factor and one of the prime reasons for differences in the output of cameras with the same pixel count CCD. The process should not be confused with the interpolation method used by some manufacturers to achieve bitmap files with a resolution greater than their true optical resolution (the resolution of their CCD array). This method - more accurately referred to as resampling - adds pixels using information already present, and although it increases the effective resolution, it does so at the cost of a reduction in sharpness and contrast. It works by quantifying pixels and qualifying them according to the common traits they possess. In place of the standard interpolation, in which pixels are copied and pasted to create larger images, Some cameras employ a software enlargement technique which it is claimed produces results better than can be achieved by conventional interpolation. This copies and pastes pixels - according to where the enlargement software thinks they are needed to make lines, shapes, patterns and contours - to create larger images.

Another limiting factor is the image compression routines used by many digital cameras to enable more images to be stored in a given amount of memory. Some digital camera store images in a proprietary format, requiring the manufacturer's supplied software for access, but most digital cameras compress and save their images in the industry-standard JPEG or FlashPIX formats, readable on almost every graphics package. Both use slightly lossy compression leading to some loss of image quality. However, many cameras have several different compression settings, allowing the user a trade-off between resolution quality and image capacity, including the option to store images in with no compression at all ("CCD raw mode") for the very best quality.

Features

A color LCD panel is a feature that is present on virtually all modern digital cameras. It acts as a mini GUI, allowing the user to adjust the full range of settings offered by the camera and is an invaluable aid to previewing and arranging photos without needing to connect to a PC to do so. Typically this can be used to display a number thumbnails of the stored images simultaneously, or provide the option to view a particular image full-screen, zoom in close and, if required, delete it from memory.

Few digital cameras come with a true single-lens reflex (SLR) viewfinder, where what the user sees through the viewfinder is exactly what the camera's CCD "sees"; most have the typical compact camera separate viewfinder which sees the picture being taken from a slightly different angle and suffer the consequent problems of parallax. Most digital cameras allow the LCD to be used for composition instead of the optical viewfinder, thereby eliminating this problem. On some models this is hidden on the rear of a hinged flap that has to be folded out, rotated and then folded back into place. On the face of it this is a little cumbersome - but it has a couple of advantages over a fixed screen. First, the screen is protected when not in use and, second, it can be flexibly positioned so as to allow the photographer to take a self-portrait or to hold the camera above their head whilst still retaining control over the framing of the shot. It also helps with one of the common problems in using an LCD viewfinder - viewing difficulty in direct sunlight. The other downside, of course, is that prolonged use causes batteries to drain quickly.

In a step designed to try to address this problem, some LCDs are provided with a power-saving skylight intended to allow it to be used without the backlight. In practice, however, this is rarely practical. If there is sufficient light to allow the skylight to work, the chances are that it will also render the LCD unusable.

Digital cameras are often described as having lenses with equivalent focal lengths to popular 35mm-camera lenses. In fact, most fixed-length lenses on digital cameras are auto-focus and have focal lengths around 8mm; these provide equivalent coverage to a standard film camera - somewhere between wide-angle and normal focal length - because the imaging CCDs are so much smaller than a frame of 35mm film. Aperture and shutter speed control are also fully automated with some cameras also allowing manual adjustment. Although optical resolution is not an aspect that features greatly in the way digital cameras are marketed, it can have a very important role in image quality. Digital camera lenses typically have an effective range of up to 20ft, an ISO equivalency of between 100 and 160 and support shutter speeds in the 1/4 of a second to 1/500th of a second range.

Digital cameras offer two distinct varieties of zoom feature: optical zoom and digital zoom. Optical zoom works in much the same way as a zoom lens on a traditional camera. Produced by the lens system, it is the magnification difference between minimum and maximum focal lengths. Importantly, in digital cameras this magnification occurs before an image is recorded in pixels. Digital zoom, on the other hand, is arguably little more than a marketing gimmick.

By the early 2000s many digital cameras came equipped with motorized optical zoom lenses which provided an effective range from wide-angle to telephoto. These generally come in a range between 3x and 10x, but it can be higher. The "times" notation can be confusing, with "3x", for example, having a different precise meaning for different cameras. This is because the actual focal length of a digital camera relates to the size of its sensor. Digital camera specifications therefore generally also cite a "35mm equivalent" lens rating. A 3x zoom lens is the standard offering and generally implies an "equivalent" focal length of some range between 35mm and 140mm. Some cameras have a gradual zoom action across the total focal range, others provide two or three predefined settings.

Digital zoom is nothing more than the cropping of the middle of an image by a digital camera's software. When an image that has been digitally zoomed 2x is reproduced, either on a display monitor or by being printed, it will effectively be viewed at half its original resolution. A more sophisticated from of digital zoom uses the digital camera's software to interpolate the cropped image back to its original resolution. In this event, fewer of the original pixels are used to represent the enlarged image, which will appear less sharp as a result. Some digital cameras provide a digital zoom feature as an alternative to an true optical zoom, others provide it as an additional feature.

For close-up work, a macro function is often provided, allowing photos to be taken at a distance as close as 3cm but more typically supporting a focal range of around 10-50cm. Some digital cameras even have swiveling lens units, capable of rotating through 270 degrees and allowing a view of the LCD viewfinder panel regardless of the angle of the lens itself.

Every digital camera has a fully automatic mode metering that allows a user to simply point and shoot. However, in common with traditional film cameras, they also offer a number of different ways of controlling the exposure of an image. A good exposure will result in an image that has balanced contrast and brightness, with no areas that are too bright and washed out or too dark which also creates loss of detail. Center weighted metering is the system used by many digital cameras to measure the correct exposure. With this system, the camera measures the amount of light mostly around the center area of the lens and less towards the edges. For many situations this works well, but in some lighting situations, center weighted metering can produce poorly exposed photos. If the scene to be photographed has light areas and dark areas, for example in the shade of trees on bright sunny days with lots of sunlight and shadowed areas, center weighed metering will often either overexpose the bright sections, or underexpose the dark sections. Some digital cameras offer matrix type metering systems, which break the scene into several areas and measures each individual area's exposure. This results in an image with a balanced exposure throughout. Spot metering is another option included on some digital camera models. This measures the exposure at a small, precise portion in the center of the lens, allowing the user to ensure perfect exposure on a particular section of the scene.

Programmed auto-exposure modes keep the basic exposure settings automatic while providing manual access to other camera settings. Some offer aperture- and shutter-priority modes which allow the user to set the f-stop or shutter speed, and then automatically calculate the other settings needed to expose an image correctly.

Some cameras provide a manual exposure mode, allowing the photographer a significant degree of artistic license. Typically, four parameters can be set in this mode: white balance, exposure compensation, flash power and flash sync. Different types of light (outdoor, fluorescent, and so on) will have an impact on the colors in images. White balance provides a means to correct for the effect of the lighting conditions, such as sunny, cloudy, incandescent or fluorescent. Exposure compensation alters the overall exposure of the shot relative to the metered "ideal" exposure. This feature is similar to that a SLR cameras, allowing a shot to be intentionally under- or over-exposed to achieve a particular effect. A flash power setting allows the strength of the flash to be incrementally altered and a flash sync setting allows use of the flash to be forced, regardless of the camera's other settings.

Some cameras offer what is referred to as "automatic exposure bracketing". With this, several frames are shot when the shutter is released, each at a different exposure setting. The exposure that gave the best result can then be selected.

Most digital cameras offer a number of image exposure timing options. One of the most popular is a burst mode that allows a number of exposures to be taken with a single press of the shutter. The speed and number of sequential shots that can be captured in a burst is dependent on the amount of internal memory the camera possesses, the image size selected and the degree of compression applied to the photos. Cameras with fast burst rates - specified as a fps rate - generally have a large amount of "buffer memory", which is used as a temporarily store prior an image being processed and written to the camera's primary image storage medium. By the early 2000s, the capability to shoot up to 15 shots in a burst at rates between 2 and 6 fps was fairly typical.

Also common is time-lapse, which delays multi-picture capture over pre-selected interval. Other examples are the ability for four consecutive shots to each use only a quarter of the available CCD array, resulting in a single frame with four separate images stored on it and to take multiple exposures at a preset delay interval, tiling the resulting images in a single frame.

Features allowing a variety image effects are becoming increasingly common. For example, a user may have the option to select between monochrome, negative and sepia modes. Apart from their use for artistic effect, the monochrome mode is useful for capturing images of documents for subsequent optical character recognition (OCR). Some digital cameras also provide a "sports" mode - which adds sharpness to the captured images of moving objects - and a "night shooting" mode which allows for long exposures.

Panoramic modes differ in their degree of complexity. At the simpler end of the spectrum is the option for a letterbox aspect image that simply trims off the top and the bottom edges of a standard image - taking up less storage space in the process. More esoteric is the ability to produce pseudo-panoramic shots by capturing a series of images and then combining them into a single panoramic landscape using special-purpose software.

A self-timer is a common feature, typically providing a 10-second delay between the time the shutter is activated and when the picture is taken and all modern day digital cameras have a built-in automatic flash, with a manual override option. The best have a working range of up to 12ft and provide a number of different modes, such as auto lowlight and backlight flash, fill flash for bright lighting shadow reduction, force-off for indoor and mood photography and red-eye reduction. Red-eye is caused by light reflected back from the retina, which is covered in blood vessels. One system works by shining an amber light at the subject for a second before the main burst of light, causing the pupil to shrink so that the amount of red light reflected back is reduced.

Another feature commonly available with film cameras that is now available on their digital counterparts is the ability to watermark a picture with a date and time, or indeed some other chosen text. And that's not all. The recent innovation of built-in microphones provides for sound annotation, in standard WAV format. After recording, this sound can be sent to an external device for playback, or played back on headphones using an ear socket. Some cameras even offer an audio made that effectively allows it to be used as a voice recorder.

A couple of other features which demonstrate the digital camera's close coupling with other aspects of PC technology are a function that allows thumbnail images to be emailed directly by camera-resident software and the ability to capture short video clips that can be stored in MPEG-1 format. Some cameras record silent video only and limit the length of the clips; others sound with the video and allow the clip to be as long as the camera is capable of saving to its storage media.

Borrowing from technology developed for their video camcorder brethren, some digital cameras feature image stabilization systems. This is particularly useful when used in conjunction with high powered zoom lenses, when it can be very difficult to keep the camera still enough to create a clear image, especially in low light situations and when using a slow shutter speed. Image stabilization is employed to help overcome the effects small movements of the camera.

Higher-end models also provide support for two memory cards and features more commonly associated with SLR-format cameras - such as detachable lenses and the ability to drive a flash unit from either the integrated hot-shoe or an external mount. Indeed, by early 2000 a number of major manufacturers - including Nikon and Kodak - were preparing to follow rival Minolta's lead in pushing digital cameras into the mainstream professional market by offering single lens reflex technology at "affordable prices". While the differential between professional and consumer models remains significant, it decreased dramatically during the late 1990s - as has the gap between digital cameras in general and their analogue counterparts.

PIM technology

In 1998, the Photographic Industry Association - comprising most of the world's digital camera manufacturers - came up with a set of standards called the Design Rule for Camera File System (DCF). This defined color parameters for digital camera images which took into account the limited color range supported by the WWW and computer display monitors. This digital camera image "target color space" was, in fact, identical to the sRGB color space originally developed by Microsoft Corporation and Hewlett Packard in the mid-1990s. The implications of this were that after being shot, an image's hues were compressed by a digital camera to make them fit within the DCF-defined color spectrum.

The DCF color standard worked well enough until early in the new millennium. However, by then the sRGB color space was not as large, nor as rich in color as the spectrum available on even the inexpensive photo-printers of the day. Users were thus being deprived of the opportunity to produce output with the nuances of color their equipment was capable of.

In early 2001 Epson unveiled a solution to this problem, in the shape of its Print Image Matching (PIM) technology. PIM works within the structure of DCF, while allowing devices with extended color capabilities to print the greater spectrum of color captured by a digital camera. It does this by getting a digital camera to store the complete color information for a captured image before it is converted to the DCF standard. When images are output to a printer the PIM printer driver software reads the associated PIM data, thereby allowing them to be reproduced with the extended range of color. PIM doesn't interfere with a digital camera's operation in any way. Specifically, it has no impact on image processing - and therefore shot-to-shot - time. Indeed, users can turn PIM off if its not required. In the event that images shot with PIM enabled are subsequently transferred to software applications or printers that don't support PIM, the PIM data is simply ignored.

PIM can be viewed as both an open and adaptable standard. Open inasmuch as the technology is available to any printer manufacturer who chooses to license it and adaptable since it can evolve as CCD sensors and printers improve, ensuring that the color fidelity of printed images will continue to be preserved in the future. It also offers the prospect of digital camera manufacturers being able to add user-selectable image enhancements for use at picture-taking time, allowing photographers to preset the intensities of such controls as contrast, color balance, highlight point, shadow point, brightness, saturation, and sharpness to suit their personal preferences.