Canon EOS Digital Rebel XT | PMA Report 2005
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PMA 2005 report | Canon EOS Digital Rebel XT PMA 2005 Report - LetsGoDigital Online Magazine
PMA Report 2005
PMA 2005 | Canon EOS Digital Rebel XT Camera Introduction

Canon EOS Digital Rebel XT CMOS sensor explanation : The Canon EOS Rebel XT features a new CMOS sensor designed and built in-house by Canon. A number of innovations proven in Canon's professional series D-SLR cameras help to ensure the sensor’s image quality is second to none. Canon's history with sensor development stretches back to 1987, when it began using the BASIS sensor for its auto focus systems. Continued R&D in the field led to the 2000 release of the Canon EOS D30: the first commercialisation of CMOS for full colour image capture.

Encouraged by the camera's success, Canon continued to refine and develop the renegade technology. Having overtaken a number of other sensor technologies along the way, CMOS now forms the basis for the world's most popular digital SLR cameras. Before Canon could launch its latest range of digital EOS cameras, the Canon EOS-1Ds Mark ll, EOS-1D Mark ll, EOS 20D and Canon Rebel XT, its engineers had to solve the big problem with small pixels, how to deal with the noise that smaller pixels would inevitably create.

Canon EOS Digital Rebel XT CMOS Sensor
Canon EOS Digital Rebel XT CMOS Sensor

The challenge: Canon's latest EOS range features four entirely new imaging sensors, each with a greater number of pixels than its respective predecessor. To squeeze more pixels onto the same sized sensor, the company's engineers had to make the pixels smaller. The problem with smaller pixels is that they are less sensitive to light and greater signal amplification is required to make it readable. This amplification can generate unwanted noise. To control this noise, Canon's engineers came up with an entirely new imaging sensor for each camera.

The APS-C sized sensor on the original Canon EOS Rebel already possessed 6.3 million effective pixels. Increasing the pixel count to 8.0 million without increasing the sensor size required a reduction in pixel size that would normally result in an increase in unwanted noise. To combat the problem, Canon devised and developed a new generation of on-pixel, noise-control amplifiers which were first proven on the Canon EOS-1Ds Mark ll, EOS‑1D Mark ll and Canon 20D. These new amps are designed not only to drain each pixel of residual charge before exposure, but also to reduce the signal noise that occurs when working with long exposures and exposures in low-light conditions.

Improving efficiency: The pixels housing each sensitive photodiode had to become smaller in the new high-density sensors, so Canon’s team had no option but to look at making them work more efficiently. Using a process of miniaturisation and electronic-component efficiency, the company's sensor designers reduced both the size of the parts that make up each pixel, as well as their number. By increasing the sensitive area of the photodiode, the team were left with a sensor that was much more sensitive to light, and thus less in need of signal amplification. So even though each pixel site is smaller, its receptive area is proportionally larger than before.

Canon EOS Digital Rebel XT

Sensitivity: Long exposures are read as they happen, with a dark frame subtraction method used to cancel native interference for true colours and smooth tonal rendering. Fixed-pattern and random noise reduction systems have also been improved to produce the cleanest images possible. However, Canon has not restricted its users to short exposures or sensor ISO values of moderate speeds. The new cameras allow fixed time exposures of 30 seconds, as well as an unlimited bulb mode that lets users keep the shutter open for as long as required. Chip sensitivity, too, can be expanded to the equivalent of ISO 1600 on the Canon Rebel XT Digital and ISO 3200 on Canon's EOS-1 series DLSRs.

Micro lenses: While the micro technologies described above play an enormous part in the image quality these new sensors provide, perhaps the greatest achievement in the conditioning of these highly populated chips is the efficiency with which they use ambient illumination. For smaller pixels to react more effectively to light, they need either more light or the light that exists to be channelled to them in a more direct manner.

Canon's engineers have developed a new type of micro lens for each pixel. Larger and more effective than models used in the past, these lenses have a greater wide-angle element, so are better equipped to gather light and direct it straight to the sensitive part of the pixel. As the lenses have less space between them, they are better equipped to gather more light, and allow less light to fall between receptive areas and be wasted. In effect, these lenses can gather more light in the same way a wide-aperture camera lens can. A larger front element catches a greater section of the information passed in its direction, while the domed aspect of the lenses draws and focuses the light where it matters most. As a result, the micro lenses pick up more of the light channelled to the sensor by the camera’s lens, using it to illuminate and inform the pixels of the sensor. Improving the effective brightness of the received image means the sensor is able to pass information to the processing engine with a lower degree of amplification than would otherwise be the case.

In the detail: Paradoxically, higher resolutions lead almost inevitably to the confusion of fine detail and edge colour, because the sensor’s resolution exceeds the boundaries where Bayer four-pixel layouts can deal efficiently with the level of detail the sensor can record. Without sensor designer intervention, this results in a distracting misrepresentation of extremely fine detail, 'moiré', as well as a colour fringing on edges where subjects of one colour meet the background colour of another, 'false colour'.

Canon's solution is to fit its cameras with a three-layer, low-pass filter that combats moiré and false colour noise patterns by ordering the incoming light to suit exactly its sampling abilities. The first part of this new filter is an infrared cut-off filter that stops the sensor recording wavelengths beyond the normal visible spectrum. This controls the sensor’s response to reds in general, ensuring they don't 'overexpose' and burn out. The next layers are a sandwich of a polarizer and single crystal substrates that combine to provide an accurate vertical and horizontal light wave layout to fit the characteristics of the sensor's resolution. This filter organises the light that passes through the lens into a form the high-resolution sensor can take most advantage from, creating the potential for sharper, more detailed images, while at the same time providing clear and defined subject edges that are not influenced by surrounding colours.

Canon EOS Digital Rebel XT
Micro becomes macro: If you were to study any of these cameras' sensors, you would almost certainly be unable to perceive any visible change. You would not be able to see the new micro lenses, the larger photodiodes, the smaller circuitry or the construction of the low-pass filter. To see the difference, you have to compare the images these sensors create with those captured by their immediate predecessors. Canon's principle aim with the introduction of these new cameras is to increase the amount of information saved to file. The purity of this information depends on the balance of signal and noise, and with these new sensors, Canon has broken the relationship between pitch and interference. This was achieved by developing methods to control exactly how the pixels react to light, and to determine how those pixels receive their light in the first place.

The gap widens: The pixel race will continue to drive increases in resolution. As this happens, the importance of noise avoidance technologies also increases to ensure that increases in resolution improve, rather than degrade, image quality. Manufacturers relying solely on software and firmware to deal with the greater noise generated by smaller pixels in higher density sensors will struggle, while brands purchasing off-the-shelf sensors have little say in how those components are designed. As a CMOS designer and manufacturer, Canon is uniquely placed to tailor pixel size, photosite design and sensor componentry to suit each new camera design and respective target market. The inherent barriers to entry of such a position means Canon's leadership in D-SLR is likely to continue for several years. "As a photography and imaging company, it has always been Canon's intention to control the entire light path, from lens through to printed image," says Mr Fukuchi, Senior General Manager, Camera Products Management Division, Canon Consumer Imaging Europe "Taming the power of CMOS has been critical to our success".

Canon EOS Digital Rebel XT

Canon low pass filter explanation : Certain fabrics can have a very tight, regular weave. On close inspection, this can appear as a series of alternating stripes separated by dark shadows between the weave. When the pitch of this weave is close to a digital camera sensor's pixel pitch, light from the subject will only activate selected rows of pixels. The sensor will then produce false colours, which can change with rotation or lateral movement of the camera. These false colours appear as waves on the image, known as moiré. Subjects such as distant geometric patterns are also susceptible to producing this effect.

To combat the effects of moiré, camera manufacturers sometimes utilize a low pass filter to blur incoming light. But a single filter can only blur a pattern in one direction, which leaves many false colours uncorrected. Moreover, this approach effectively lowers the camera's resolution. Canon's approach is to use a unique three-layer Canon low pass filter. The first filter polarises and separates incoming light vertically by precisely one pixel distance. In order to then split the light horizontally, the light must first have the polarisation removed. This is achieved with a second filter known as a phase plate. The third low pass filter polarises the light a second time, only this time separating it horizontally. Incoming light is therefore spread by precisely one pixel in both the vertical and horizontal directions.

This deliberate blurring has the effect of removing moiré. Unless it is compensated for, however, it also has the effect of reducing image sharpness. Performing accurate compensation, however, requires an exceptionally complex series of processor hungry algorithms, one of the reasons why competing manufacturers have difficulty tackling the moiré phenomenon. One of the Canon's great advantages is the processing 'head room' of its DIGIC II processor, which is used to perform the most sophisticated of correction routines and ensure the benefit of sensor resolution without any trade off in camera performance. The result is high resolution images with outstanding colour reproduction accuracy.

Canon EOS Digital Rebel XT

Canon DIGIC II Image processor introduction : The Canon EOS Digital Rebel XT features Canon's DIGIC II processor, proven in Canon's pro-series Canon EOS-1Ds Mark II and EOS-1D Mark II cameras. DIGIC II delivers superb image quality, responsive camera performance, faster AF, faster continuous shooting and extended battery life.

"There are three critical stages affecting digital image quality," says Brian Worley, Camera Product Manager, Canon Europe Ltd. "First the lens determines how the image is transmitted on to the sensor. Then the sensor determines how accurately the light is converted to electrical signals. The third and final stage is the image processor, which dictates how well the signals from the sensor are translated into a viewable image. You need to get each step right to make a good image. Get one of them wrong, and you ruin everything." With over 60 years of precision lens manufacture behind it, Canon has little to prove in the area of optics. With its own award winning CMOS technology Canon is also well in control of the sensor. With its dedicated DIGIC and now DIGIC II (Digital Image Core) image processors, Canon has revolutionised the way digital images are handled.

Ultra high speed: With the introduction of Canon DIGIC II, Canon brings a solution for doing the high speed calculations necessary in order to providing exceptionally accurate colour reproduction in real time. "DIGIC II operates many times faster than other image processors. It can handle more data in a shorter period to allow more complex calculations to take place," says Worley. "DIGIC II runs not on software as others do but as a piece of hardware built into the camera's circuitry. As such it is able to carry out the duties of a number of separate processing units, effectively replacing them to save both time and space." Most digital camera manufacturers face a trade-off between camera responsiveness and the amount of processing each image can receive. For high quality images, data received from the recording sensor needs a lot of attention. But working on that data to the extent required in order to optimise image quality takes time, and can hold-up the operation of the camera. To alleviate this problem, many cameras compromise image quality by using simplified processing methods, leaving off or 'dumbing down' certain processing procedures. Large and expensive buffer memories are required to store data during busy periods, to be processed later when the camera is at rest. This creates its own problems when buffer memories become full, causing cameras to freeze until some of the backlog is cleared.

"With DIGIC II, Canon has created a processor so fast it can read, process, compress and write image data back to the buffer between exposures," explains Worley. "This allows continuous shooting with the Canon Rebel XT digital camera without the camera becoming clogged with data. Moreover, each image is subject to the complex and individual processing required in order to deliver superb image quality." The image quality delivered by DIGIC II is so good that leading photographic agency Getty Images chooses to shoot JPEG rather than RAW images.

Canon EOS Digital Rebel XT

White Balance: Apart from the speed with which it clears data from the camera's buffer, the benefits of DIGIC II are most obvious in the areas of white balance (WB) adjustment. The camera uses DIGIC II's additional processing power to build an intelligent understanding of the scene to be captured by taking into account factors such as orientation and subject position. This allows more accurate calculation of auto white balance. "While other manufacturers use systems that divide the scene into hundreds to segments for white balance assessment, the DIGIC II processor in the Canon Rebel XT digital camera has the time and power to look at tens of thousands of segments to build a complex plan of how the scene is constructed. This allows the camera to distinguish between more than one type of light source in a single scene and to treat each area individually," explains Worley.

Another feature of AWB under DIGIC II is what Canon calls 'Feeling Based White Balance'. This is a system that aims to correct casts without destroying the atmosphere of the scene, allowing it to be reproduced in a realistic way. AWB systems that adjust with technical correctness can remove the warmth of a romantic meal for two, the orange/red light of the sun as it dips below the horizon. "This Feeling Based White Balance will reproduce colours closer to the way we see them in different situations, rather than striving to always create a textbook-accurate but sterile image," says Worley.

More stamina: One of the benefits that has come about with the introduction of DIGIC II is extended battery life. "As the processor is only operating for very short periods, it doesn’t use much power," explains Worley. "DIGIC II is part of the reason that the Canon EOS Rebel XT is capable of taking 600 shots on just a single battery charge."

Investment in R & D: "The development of DIGIC and DIGIC II has been made possible by the vast amount of resources Canon has allocated to research and development over many years, and the company's position as a manufacturer of its own electronic components and circuitry," Worley explains. "By increasing its experience in the manufacture of Application Specific Integrated Circuits (ASIC) for its digital camera production line, Canon has ensured that every major piece of hardware required for the construction of the Canon EOS Rebel XT is provided in-house. This puts Canon in a special position in the digital SLR market, a company that produces every significant element that goes into its cameras, from the lenses to the electronics and internal hardware, and in the case of the Canon EOS Digital Rebel XT, the actual sensor itself."

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