It’s easy to understand the booming business that digital camera manufacturers are doing these days. The host of easy-to-use personal and business publishing applications, the dramatic expansion of the Web and its insatiable appetite for visual images, and the proliferation of inexpensive printers capable of photo-realistic output make a digital camera an enticing add-on. Those factors, combined with improving image quality and falling prices, put the digital camera on the cusp of becoming a standard peripheral for a home or buusiness PC.
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 meemory 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 ar
This entire process is very environment-friendly. The CCD orr 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 minimise the number of moving parts.
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 im
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 100×100 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.
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 ha
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 colour. Colour 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 colour, the true colour is made by averaging the light intensity of the pixels around it – a process known as colour interpolation.
Recognising a glass ceiling in the conventional charge-coupled device (CCD) design, Fujifilm have developed a new, radically di
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. Problems remain to be solved – such as noisy images and an inability to capture motion correctly – and 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.
Hitherto, 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 (4096×4096) sensor is the first image sensor of any size to be manufactured with 0.18 micron process technology – a proprietary analogue 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 4096×4096 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.
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 640×480 pixels. A year later as manufacturing techniques improved and technology progressed the emergence of “megapixel” cameras meant that the same money could buy a 1024×768 or even a 1280×960 model. By early 1999, resolutions were as high as 1536×1024 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 1800×1200. 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 2048×1536 pixels. The first consumer model 4 megapixel camera appeared in mid-2001, boasting a maximum image size of 2240×1680 pixels.
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 colour 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.
A colour 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 motorised 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 swivelling 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 centre area of the lens and less towards the edges. For many situations this works well, but in some lighting situations, centre 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, centre 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 centre 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 licence. 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 colours 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 preselected 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 stabilisation 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 stabilisation 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 hotshoe 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.
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 colour parameters for digital camera images which took into account the limited colour range supported by the WWW and computer display monitors. This digital camera image “target colour space” was, in fact, identical to the sRGB colour 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 colour spectrum.
The DCF colour standard worked well enough until early in the new millennium. However, by then the sRGB colour space was not as large, nor as rich in colour 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 colour 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 colour capabilities to print the greater spectrum of colour captured by a digital camera. It does this by getting a digital camera to store the complete colour 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 colour. 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 colour 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, colour balance, highlight point, shadow point, brightness, saturation, and sharpness to suit their personal preferences.
The fact is that a digital camera is a high drain device that uses up batteries at an alarming rate. Turning off the LCD display helps considerably as does running on AC power whenever possible – such as when transferring images to a PC or viewing images on a TV. While digital camera batteries come in all shapes and sizes, the AA format is by far the most common. However, traditional alkaline AA batteries should be relied on only in emergencies. They are simply not strong enough for a power-hungry instrument like a digital camera for more than a few dozen images.
An ordinary AA alkaline battery is typically supposed to have a capacity of 2.4Ah (amp hours). This means it should be able to deliver 1.2A for two hours before going flat. However, whilst this expression of cell capacity is fine for low-current applications – like personal cassette players – its not at all appropriate for a high-current device like digital cameras, which are capable of imposing loads of such magnitude that they can cause a battery’s voltage to drop precipitously, with the consequence that its life is greatly reduced.
Rechargeable cells are much better at handling the high output currents required by digital cameras and despite the fact that some types have a notoriously poor “shelf life” – the length of time they’ll hold a charge – they’re generally the most cost-effective option in the long run. They come in a variety of types or “families”:
• Nickel cadmium (NiCd): Probably the most robust and commonly available rechargeable battery. Good for on average 700 charge and discharge cycles, they lose about 1% of their power a day when not in use and suffer badly from “memory effect” – the accumulation of gas bubbles on the cell plates of a battery that has only been partially discharged before recharging, which causes a reduction in the plate area within the battery and thus its capacity.
• Nickel metal hydride (NiMH): Good for between 500-1000 charge and discharge cycles, NiMH batteries offer about 40% more capacity than NiCDs but at a significantly higher cost. They have a slightly worse shelf-life than NiCds but, in the main, have the important advantage of not being prone to “memory effect”.
• Lithium ion (Li-ion): Li-ion batteries offer about twice the capacity of a similarly sized NiMH battery and are good for about 500 charge and discharge cycles. However, they require their own special charger and are more expensive than other battery types. Their big advantage is a long shelf life: up to ten years. For that reason, they make great emergency spares.
Zinc-air batteries have been around for decades and have found practical use in hearing aids and pagers where their light weight and high energy density have enabled them to dominate the market for a number of years. In recent years electrical and mechanical enhancements have allowed the technology to be adapted for use in portable electronic systems.
Zinc-air battery cells are chemically similar to the everyday alkaline battery. However, in place of the magnesium dioxide paste used by the former is a thin carbon electrode used to catalyse oxygen from air for reaction with zinc. This significantly increases the energy density, giving zinc-air one of the highest energy densities for conventional battery systems.
By the early 2000s disposable zinc-air batteries had emerged as an increasingly popular back-up or emergency power source in digital photography, compatible with more than 50 digital camera models. They offered a long shelf life and sufficient power for several hours of continuous use. Packaging typically included a recloseable foil pouch that allowed the user to extend battery life after initial use, a built-in belt clip for easy carrying and a cord that allowed connection through a camera’s DC jack.
Many first-generation digital cameras contained one or two megabytes of internal memory suitable for storing around 30 standard-quality images at a size of 640×480 pixels. Unfortunately, once the memory had been filled no more pictures could be taken until they’d been transferred to a PC and deleted from the camera.
Modern digital cameras use removable storage. This offer two main advantages: first, once a memory card is full it can simply be removed and replaced by another; second, given the necessary PC hardware, memory cards can be inserted directly into a PC and the photos read as if from a hard disk. By early 1999 two rival formats were battling for domination of the digital camera arena:
• CompactFlash: First introduced in 1994 by SanDisk corporation, and based on flash memory technology, CompactFlash (CF) provides non-volatile storage that doesn’t require a battery to retain data. It’s essentially a PC Card flash card that’s been reduced to about one quarter of its original size and uses a 50-pin connection that fits into a standard 68-pin Type II PC Card adapter. This makes it easily compatible with devices designed to use PC Card flash RAM. CompactFlash cards measure 43mm by 36mm. They are available in both Type I and Type II cards, although predominately the former. A Type I card is 3.3mm thick and will operate in both a Type I and a Type II slot. A Type II card 5mm and will operate in a Type II slot only. By late 2001 maximum capacities had reached 512MB.
• SmartMedia: Originally known by the awkward acronym SSFDC (Solid-State Floppy Disk Card) when it first appeared in 1996, the Toshiba-developed SmartMedia cards are significantly smaller and lighter than CompactFlash cards, weighing 0.48g with a form factor of 45 by 37mm and a thickness of only 0.78mm. It uses its own proprietary 22-pin connection – but like its rival format is PCMCIA-ATA-compatible and can therefore be adapted for use in notebook PC Card slots. Capacities are less than for CompactFlash – 128MB was still the maximum capacity by late 2001, capable of storing 560 high-resolution (1200×1024) still photographs – and cost per megabyte is similar to that of CompactFlash.
Devices are available for both types of media to allow access via either a standard floppy disk drive or a PC’s parallel port. The highest performance option is a SCSI device which allows PC Card slots to be added to a desktop PC. CompactFlash has a far sturdier construction than its rival, encapsulating the memory circuitry in a hard-wearing case. SmartMedia has its gold-coloured contact exposed, and prolonged use can cause scoring on its surface. Its memory circuitry is set into resin and sandwiched between the card and the contact. CompactFlash can operate between temperatures of 25oC to 75oC and claims a 100-year usage life; SmartMedia can be used between 0oC to 50oC and claims that it can be written to at least 250,000 times.
With the 24-bit colour, 1800×1200 resolution images being produced by consumer models by mid-1999 occupying a massive 6.2MB – storage capacity is becoming an increasingly important aspect of digital camera technology. It is not clear which format will emerge as winner in the standards battle. SmartMedia has got off to a good start, but CompactFlash is used in PDAs as well, and this extra versatility might prove an important advantage in the long run.
By the end of 1999 a third memory technology had emerged, in the shape of Sony’s Memory Stick. Smaller than a stick of chewing gum and initially available with a capacity of 32MB, Memory Stick is designed for use in small AV electronics products such as digital cameras and camcorders. It’s proprietary 10-pin connector ensures foolproof insertion, easy removal, and reliable connection and its unique Erasure Prevention Switch helps protect stored data from accidental erasure. Capacities had risen to 128MB by late 2001, with the technology roadmap for the product going all the way up to 1GB.
By the end of 2001 the digital camera memory market share for CompactFlash, SmartMedia and Memory Stick was estimated to be around 44%, 33% and 6% respectively.
Some higher-end professional cameras use PCMCIA hard disk drives as their storage medium. Although they consume no power once images are recorded, and have much higher capacity than flash memory (a 170MB drive is capable of storing up to 3,200 images “standard” 640 by 480 images), the hard disk option has some disadvantages. An average PC Card hard disk consumes around 2.5W of power when spinning idle, more when reading/writing, and even more when spinning up. This means it’s impractical to spin up the drive, take a couple of shots and shut it down again; all shots have to be taken and stored in one go, and even then the camera’s battery will last a pitifully short length of time. Fragility and reliability are also a major concern. The moving parts and extremely tight mechanical tolerances to which hard drives are built make them inherently less reliable than solid-state media.
With the resolution of still digital cameras increasing apace and the emergence of digital video cameras, the need for flexible, high-capacity image storage solutions has never been greater. In 1999 Iomega launched an innovative removable storage device intended for use in digital cameras as well as notebook and handheld devices. The battery-powered Clik! drive supports the PC Card interface and provides a capacity of 40MB on its 10g, 50x50mm media. It comes complete with an adapter, allowing the transfer of images from CompactFlash and SmartMedia cards to its significantly cheaper 40MB magnetic disk media. The following year Agfa’s ePhoto CL30 Clik! became the first digital camera to use Clik! disks as its primary mode of storage.
Mid-1999 also saw IBM enter the fray with the launch of the world’s smallest hard disk drive, its revolutionary Microdrive. The Microdrive uses a single one-inch diameter platter that weighs just 16g and spins at 4,500rpm and . Using the Type II CompactFlash interface, the new device – initially available in capacities of 170MB and 340MB – took CF-style storage into a new dimension. By the end of 1998 the largest available CompactFlash memory card held just 64MB and in mid-2000 digital cameras rarely came with more than an 8MB card. In 2000 IBM announced 512MB and 1GB versions of the Microdrive. As well as their increased capacity, these second-generation drives also boasted improved sustained data rates – and thus extended battery life – and a non-operating shock rating that had been improved by 50%, from 1,000 G to 1,500 G.
One of the major advantages of a digital camera is that it is non-mechanical. Since everything is digital, there are no moving parts – and subsequently a lot less that can go wrong. However, this didn’t deter Sony from the taking a step which can be viewed as being imaginative and retrograde at the same time – including an integrated 3.5in floppy disk drive in some of its Mavica range of digital cameras. Whilst the company’s claim that the floppy disk remained the storage media of choice for many was debatable, the fact that floppy disk media was universally compatible, cheap and readily available was undeniable. It was also easy to use – no hassles with connecting wires or interfaces. While the integrated drive obviously added both weight and bulk to a device that’s usually designed to be as compact as possible – it is true that some users actually prefer designs that lend themselves to a double-handed grip. Its potential unreliability was addressed to some extent by the provision of a “Whole Disk” copy facility that allowed users to easily make copies of disks. This copied images from the original disk to the camera’s internal memory and thence to a second disk.
In the second half of 2000 Sony updated its innovative approach with the introduction of a Mavica model which stored images on 8cm/185MB CD-R media. A so-called “mini CD” provides sufficient capacity to store around 300 640×480 resolution images using JPEG compression. In early 2001 the concept advanced a step further with support for mini rewritable media as well as write-once CD-R media. Performance implications that would have been unacceptable for digital camera applications made certain (write-time) trade-offs unavoidable. Notwithstanding the constraints imposed, the primary benefit of CD-RW media – it’s reusability – is fully realised. As well as being able to delete images one at a time -starting with the most recent and working backward – users have the option to erase an entire disc via a “format” function.
Despite the trend towards removable storage, digital cameras still allow connection to a PC for the purpose of image downloading. Until the late 1990s the principal method of transfer was via a conventional RS-232 serial cable at a maximum speed of 115Kbit/s. However, since then USB connectivity has become the norm, with most manufacturers bundling cameras with the necessary cables and driver software. In the past some professional models offered a fast SCSI connection. However, FireWire has subsequently become the technology of choice for this market sector.
USB is also exploited by a method of image transfer that emerged in the early 2000s and that saves the bother of having to connect a camera to a PC at all. These “media readers” are available for all the common media types – CompactFlash, SmartMedia, MemoryStick, MultiMedia Card etc. – and simply plug in to a USB port, either directly or via an extension cable. They’re also available in combo units, capable of reading multiple media formats. They draw power from the USB port and so don’t waste a camera’s precious batter power. They vary in speed, the fastest being capable of data transfer rates approaching 1 MBps. For those without USB, adapters are also available that allow a CompactFlash or SmartMedia card to be inserted into a special floppy disk like device that works from a standard 3.5in 1.44MB floppy disk drive.
Supplying a digital camera with TWAIN drivers that allow the user to simply download images to a standard image editing application has also becoming standard practice. Some digital cameras provide a video-out socket and S-Video cable to allow images to be displayed directly to a projector, TV or VCR. Extending the “slide show” capability further, some allow images to be uploaded to the camera, enabling it to be used as a mobile presentation tool.
An increasing number of digital cameras have the ability to cut out the computer and output images directly to a printer. But without established interface standards, each camera requires a dedicated printer from its own manufacturer. As well as the more established printer technologies, there are two distinct technologies used in this field: thermo autochrome and dye sublimation.
Digital vs film
Despite the massive strides it has made in recent years, the conventional wisdom remains that though digital cameras offer advantages in term of flexibility, when it comes to picture quality they still fall a significant way behind that of a traditional camera and film. However, since this assertion involves the comparison of two radically different technologies, it is worth considering more closely.
The first step is to consider resolution. Whilst its easy to state the resolution of a digital camera’s CCD, expressing the resolution of traditional film in absolute terms is more difficult. Assuming a capture resolution of 1280×960 pixels, a typical 1999 model digital camera is capable of producing a frame size of just over 1.2 million pixels. A modern top-of-the-range camera lens is capable of resolving at least 200 pixels per mm. Since a standard 100ASA 35mm negative is 24x36mm, this gives an effective resolution of 24x200x36x200 = 34,560,000. This resolution is rarely achieved in practice and, indeed, rarely required. However, on the basis of resolution, it is clear that digital cameras still have some way to go before they reach the level of performance as their conventional film camera counterparts.
However, this is only part of the answer. The next factor to consider is colour – and here digital cameras have an advantage. Typically, the CCDs in digital cameras capture colour information in 24 bits per pixel. This equates to 16.7 million colours and is generally considered as being the maximum number the human eye can perceive. On its own this doesn’t constitute a major advantage over film. However, unlike the silver halide crystals in a film, a CCD captures each of the three component colours (red, green and blue) with no bias. Photographic film tends to have a specific colour bias – dependent on the type of film and, to a certain extent, the manufacturer – and this can have an adverse effect on an image, according to its colour balance.
However, its also its silver halide crystals that give photographic film its key advantage. While the cells on a CCD are laid out in rows and columns, the crystals on a film are, to all intent and purposes, randomly arranged with no discernible pattern. As the human eye is very sensitive to patterns, it tends to perceive the regimented arrangement of the pixels captured by a CCD very easily, particularly when adjacent pixels have markedly different tonal values. Magnify photographic film, and though the dots will be discernible, there will be no apparent regularity. Its for this reason that modern inkjet printers use a technique known as “stochastic dithering”, which adds a random element to the pattern of the ink dots in order to smooth the transition from one tone to the next. Photographic film does this naturally, so the eye perceives the results as less blocky when compared to digital stills.
There are two possible ways around this problem for digital cameras. Manufacturers can either develop and build models that can capture a higher resolution than the eye can perceive, or they can build in dithering algorithms that alter an image after it has been captured by the CCD. Both of these options have downsides however, such as increased file sizes and longer processing times.
In 2002 the prospect of truly affordable film-quality digital cameras was given a massive boost when – after five years of research and development – Foveon Corporation unveiled a digital camera imaging sensor which the company claimed was capable of obviating 35mm film.
In conventional digital cameras systems colour filters are applied to a single layer of photo-detectors in a tilted mosaic pattern. The filters let only one wavelength of light – red, green or blue – pass through to any given pixel, allowing it record only one colour. As a result, typical mosaic sensors capture 50% of the green and only 25% of each of the blue and red light. The approach has inherent drawbacks, no matter how many pixels a mosaic-based image sensor might contain. Since they only capture one third of the colour, mosaic-based image sensors must rely on complex processing to interpolate the two-thirds they miss. Not only does this slow down the speed of image rendering, interpolation also leads to colour artefacts and a loss of image detail. Some cameras even intentionally blur pictures to reduce colour artefacts.
Foveon’s new CMOS image sensor uses the company’s revolutionary X3 technology to capture up to three times more information per pixel than modern-day digital cameras at similar megapixel resolutions. The X3 image sensors accomplish this by using three layers of photodetectors embedded in silicon. The layers are positioned to take advantage of the fact that silicon absorbs different colours of light at different depths, so one layer records red, another layer records green and the remaining layer records blue. This means that for every pixel on a Foveon X3 image sensor, there’s actually a stack of three photodetectors. The result is a sensor capable of capturing red, green, and blue in each pixel location – in essence, the first full-colour digital camera image sensor.
Foveon’s X3 technology not only leads to better pictures, but better cameras too. In fact, it opens the door to an entirely new breed of camera, one that can switch seamlessly between still photography and digital video, without sacrificing the quality of either. Because Foveon X3 image sensors capture full colour at every pixel location, those pixels can be grouped together to create larger, full-colour “super pixels.” This capability, called Variable Pixel Sizing (VPS), marks another first in digital photography.
With VPS, the signals from groups of pixels can be combined so the camera reads them as one. For example, a 2300×1500 image sensor contains more than 3.4 million pixels. But if VPS is used to group those pixels into 4×4 blocks, the image sensor would appear to have 575×375 pixels, each of them 16 times larger than the originals. The size and configuration of a pixel group is variable – 2×2, 4×4, 3×5, etc. – and is controlled through sophisticated circuitry integrated into Foveon X3 image sensors.
The grouping of smaller pixels into larger pixels increases the signal-to-noise ratio. This allows the camera to take full-colour pictures in low-light conditions with reduced noise. Using VPS to reduce the resolution also allows the sensor to run at higher frame rates, accelerating the rate at which pictures can be taken. These gains in speed and sensitivity offer other benefits such as an improved focusing system.
VPS also makes it possible to switch from high-quality still photography to outstanding digital video, enabling the development of the first cameras with true dual-mode functionality. Hitherto, cameras attempting to accommodate both still and video functions have had to sacrifice performance in one mode to do the other well. The unique design of Foveon X3 image sensors enables them to handle both functions without compromise.