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.
Encouraged by the camera's success, Canon continued to refine and develop the renegade technology. The challenge: Canon's latest EOS range features four entirely new imaging sensors, each with a greater number of pixels than its respective predecessor.
The APS-C sized sensor on the original Canon EOS Rebel already possessed 6.3 million effective pixels. 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.
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. 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. Micro becomes macro: If you were to study any of these cameras' sensors, you would almost certainly be unable to perceive any visible change. To combat the effects of moiré, camera manufacturers sometimes utilize a low pass filter to blur incoming light. 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. 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.
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.
More stamina: One of the benefits that has come about with the introduction of DIGIC II is extended battery life. This giant, slow moving grasshopper’s bright orange, yellow and red colors are a warning that it contains toxins and will make any potential predator sick. 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.
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. To squeeze more pixels onto the same sized sensor, the company's engineers had to make the pixels smaller.
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. 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. Fixed-pattern and random noise reduction systems have also been improved to produce the cleanest images possible. 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. 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.
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'. The first part of this new filter is an infrared cut-off filter that stops the sensor recording wavelengths beyond the normal visible spectrum. 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.


As this happens, the importance of noise avoidance technologies also increases to ensure that increases in resolution improve, rather than degrade, image quality. On close inspection, this can appear as a series of alternating stripes separated by dark shadows between the weave. But a single filter can only blur a pattern in one direction, which leaves many false colours uncorrected.
DIGIC II delivers superb image quality, responsive camera performance, faster AF, faster continuous shooting and extended battery life. 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 is a system that aims to correct casts without destroying the atmosphere of the scene, allowing it to be reproduced in a realistic way. It supports Full HD 1080p video recording at up to 30fps and 3.7 fps continuous shooting up to approximately 34 JPEGs or approximately 6 RAW. Canon's history with sensor development stretches back to 1987, when it began using the BASIS sensor for its auto focus systems. The problem with smaller pixels is that they are less sensitive to light and greater signal amplification is required to make it readable. 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.
However, Canon has not restricted its users to short exposures or sensor ISO values of moderate speeds.
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.
This controls the sensor’s response to reds in general, ensuring they don't 'overexpose' and burn out. To see the difference, you have to compare the images these sensors create with those captured by their immediate predecessors.
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. 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.
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. It can handle more data in a shorter period to allow more complex calculations to take place," says Worley.
The T3i offers ISO up to 6400, LiveView mode, enhanced metering with a 63-zone, Dual-layer metering system for accurate metering between exposures, and 9-point AF system as well as Basic+ function, Multi-Aspect function and Creative Filters.
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.
So even though each pixel site is smaller, its receptive area is proportionally larger than before.
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. In effect, these lenses can gather more light in the same way a wide-aperture camera lens can. 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.
Canon's principle aim with the introduction of these new cameras is to increase the amount of information saved to file.


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 sensor will then produce false colours, which can change with rotation or lateral movement of the camera.
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 third and final stage is the image processor, which dictates how well the signals from the sensor are translated into a viewable image. To control this noise, Canon's engineers came up with an entirely new imaging sensor for each camera. 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. 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. 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. 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.
The inherent barriers to entry of such a position means Canon's leadership in D-SLR is likely to continue for several years. The first filter polarises and separates incoming light vertically by precisely one pixel distance.
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.
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. 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.
In order to then split the light horizontally, the light must first have the polarisation removed. 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. 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. The third low pass filter polarises the light a second time, only this time separating it horizontally.
With its dedicated DIGIC and now DIGIC II (Digital Image Core) image processors, Canon has revolutionised the way digital images are handled. To alleviate this problem, many cameras compromise image quality by using simplified processing methods, leaving off or 'dumbing down' certain processing procedures. Incoming light is therefore spread by precisely one pixel in both the vertical and horizontal directions. 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.



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