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Canon EOS Digital Rebel XTi Digital SLR
Canon EOS Digital Rebel XTi Digital Technology
At the heart of the Rebel XTi SLR's high-resolution image capability is its large, single plate, CMOS color image sensor. Designed and manufactured by Canon specifically for the Rebel XTi SLR, this highly responsive sensor's 10.1 million pixels are fractionally smaller than the pixels in the 8 megapixel Rebel XT model, yet deliver markedly improved resolution, enabling the sensor to capture more image information. This results not simply in the ability to generate larger images, but also permits details from cropped images to be rendered with higher image quality than cropped images from lower resolution sensors.
The EOS Digital Rebel XTi camera maintains the top-tier technologies that have helped power the EOS line of digital SLRs to the forefront of the industry and the marketplace. These include Canon's power efficient DIGIC II Image Processor. Canon's DIGIC II image processor is without question, the most powerful imaging processor that the company has ever put into a digital camera. In addition to its image quality improvements, the DIGIC II chip is also responsible for speeding up camera performance in areas such as start-up time, autofocus calculation, card-writing speed, direct printing and data transfer speed through the USB 2.0 High Speed interface.
Newly Developed, High-Resolution 10.1 Megapixel CMOS Sensor
The all-new Canon CMOS sensor in the EOS Digital Rebel XTi gives this camera the highest overall performance in its class. Like all other Canon CMOS sensors, this unit is designed and manufactured by Canon using semiconductor manufacturing equipment which was also designed and manufactured by Canon. This fully-integrated design and manufacturing system makes Canon unique in the photo industry. Canon has now produced over 5 million CMOS sensors, all for use exclusively in Canon DSLR cameras. The experience this achievement represents, in addition to constant R&D efforts, has enabled Canon to refine and improve its image sensors to their current remarkable level.
CMOS sensor |
The image sensor in the Digital Rebel XTi measures 22.2 by 14.8 mm. Each of its 10.1 million pixels measures 5.7 µm square. Its predecessor, the Digital Rebel XT, had 8.0 megapixels, each measuring 6.4 µm square. Several improved technologies help the XTi to continue the Canon tradition of offering a wide range of ISO settings (100-1600), all of which are highly usable and characterized by very low noise despite the smaller pixel size that is a function of higher resolution. First, the spacing between the on-chip microlenses is now about half of the Digital Rebel XT's. This new configuration gathers light much more effectively, loses less light between the microlenses, and improves light convergence. Second, a higher percentage of each pixel's surface area is sensitive to light. Third, the output amplifiers have been optimized, lowering noise. Last, the second-generation, on-chip noise reduction circuit minimizes random noise and removes fixed-pattern noise. As a result, the XT and the XTi have the same signal-to-noise ratio and equivalent dynamic ranges despite the smaller pixels of the XTi. Finally, the 3 frames-per-second continuous shooting speed of the XT has been maintained notwithstanding the larger file size of the XTi. The combination of two-channel readout from the sensor, increased processor clock speed and improved performance of the output amps is responsible for this accomplishment.
The infrared-blocking, optical low-pass filter is an integral part of the sensor package, and is divided into two individual components for the first time. Low-pass filter #1 is part of the Self Cleaning Sensor Unit. Its front surface is coated with a dichroic mirror that reflects infrared wavelengths. LPF #1 also separates the subject image into two images horizontally. The phase plate, or phaser layer, converts the images separated by LPF #1 from linear polarization to circular polarization and prepares these polarized components to be correctly separated into four square images by low-pass filter #2. The hybrid infrared-absorption glass reflects and absorbs infrared light, effectively suppressing red ghosting and color casts caused by reflections on the sensor surface. The subject image divided into two images horizontally by the low-pass filter is separated vertically into two images by low-pass filter #2. The image is thereby properly separated into four square images. The image separation width for the vertical and horizontal images is optimized for the sensor pitch, minimizing color artifacts and moire caused by minute horizontal- and vertical-line patterns. As in the case of the Canon EOS 5D, low-pass filter #2 also serves as the CMOS sensor package's cover glass, eliminating a relatively expensive glass cover and reducing cost.
Self Cleaning Sensor Unit |
Extraordinary New Dust Reduction System
Most modern camera stores now have a department devoted to dust removal tools: brushes sized to match sensors, electronic brushes, brushes that spin, solutions -- in liquid and tablet form -- for cleaning brushes, swabs of many types, wipes, grabbers, sticks, sleeves, liquids (ethanol, methanol and mysterious agents), rubber bulbs both large and small, chamber cleaning swabs and solutions, and, of course, more. The reason for this profusion is that digital SLRs with interchangeable lenses are susceptible to dust particles entering the camera when the lens is changed. The dust can settle on the sensor and leave a spotty mess on photographs, especially in less variegated areas like skies and clouds. The dust needs to be removed periodically, but touching a sensor is regarded (properly) by most people, no matter how skilled or prepared, as something to be avoided until or unless it is absolutely unavoidable.
In the Digital Rebel XTi, Canon offers an astute new approach to the dust problem: a self-cleaning sensor unit and Dust Delete Data software. Traditional manual sensor cleaning remains an option, of necessity, but it now needs to done much, much less often, if at all, depending on shooting conditions and photographers' habits. The overall strategy is one of, first, neither generating nor attracting dust. The shutter unit generates minimal dust; the body cap is now made of a material which minimizes dust caused by normal wear and rubbing, and the low-pass filter is treated with an anti-static charge process to prevent static-charged dust from adhering to it.
In the course of developing this unit, Canon engineers analyzed the causes, types and sizes of visible dust. The dust becomes most noticeable at small apertures, so they studied what sizes of dust were the worst offenders at small apertures. The Self Cleaning Sensor Unit was then designed to eliminate those types of dust most effectively. Lowpass filter #1 on the front of the sensor is attached to an ultrasonic vibrating unit driven by a piezoelectric element. When LPF #1 is subjected to ultrasonic vibrations, the adhering dust is shaken off the surface.
The removed dust then sticks to an absorbent material ringing the low-pass filter. The sensor unit also has an internal O-ring around the perimeter to keep out dust. The area between LPF #1 and the CMOS sensor is thus sealed; dust cannot enter from the side. Instead of a dust-removal system which uses a vibrating glass, the Digital Rebel XTi uses its low-pass filter to vibrate directly, shaking off dust. Therefore, the optical performance is not degraded by an extra sheet of glass, and compactness is maintained. The camera body size is virtually the same as that of the Digital Rebel XT.
The default setting for the self-cleaning sensor has the cleaning unit operating for 1 sec. when the camera's power switch is turned on or off. Dust is therefore removed before shooting and after shooting is completed, the latter instance because dust is more difficult to dislodge after it has been stuck for a long time. Self-cleaning can be enabled or disabled manually. The self-cleaning system can also be activated manually with the menu item Clean now. Note that shooting priority still holds. If you press the shutter button halfway or press the menu button during the self-cleaning operation (in either the Auto or Manual modes), the cleaning operation will stop and the camera will be ready to shoot. Pressing the shutter button during the internal processing startup sequence (immediately after the 0.2 second startup) will make shooting possible at that moment and sensor cleaning will not occur.
Dust Delete Data Detection
Two kinds of dust are less likely to be moved by the accelerations of the vibrating LPF: small and light particles and the dreaded sticky stuff. In the event that the Self Cleaning Sensor Unit cannot remove all the dust, the Dust Delete Data (the size and position of dust too small or too sticky to remove with the Self Cleaning Sensor Unit) is obtained and appended to the image so that Digital Photo Professional Ver. 2.2 can erase the remaining dust spots on the image automatically. The shadows created by the dust particles adhering to low-pass filter #1 are detected by the imaging sensor. The most troubling dust particles, mostly the larger ones, are then singled out and their location coordinates are obtained as Dust Delete Data. This data is attached to the image. After the shutter is released, it takes about 6 sec. to obtain the Dust Delete Data.
DPP Ver. 2.1 had the copy stamp tool for manually selecting and erasing round dust spots. This feature has been improved with the DPP Ver. 2.2 which can use the Dust Delete Data to erase the dust spots automatically. When an image contains Dust Delete Data, the automatic dust spot erasing process activates by starting the copy stamp tool and pressing the [Apply Dust Delete Data] button. With DPP Ver. 2.2, the dust spots at the locations specified by the Dust Delete Data are detected and erased if doing so is deemed effective. This modest bit of artificial intelligence will be appreciated by anyone who has done a little spotting or retouching because certain things (and it's often difficult to figure out just which ones) look worse after they've been repaired and are better off left alone.
DIGIC II Image Processor
Canon's most powerful SLR imaging engine, DIGIC II provides high-precision and high speed, natural color reproduction including high-saturation, bright subjects, improved auto white balance, along with increased white balance correction control and white balance bracketing. The DIGIC II's algorithm yields improve color reproduction, especially with high-saturation, bright subjects, improved auto white balance precision for a wide range of scenes and light sources, and wider dynamic range in highlight areas.
Fast 3 Frames-per-Second 27-Frame Burst and 0.2 Second Startup Time
While the Digital Rebel XTi offers the same fast 3 frames per second as the Digital Rebel XT model, the burst rate in Large/Fine JPEG and Raw settings has nearly doubled, from 14 eight-megapixel JPEG images and five eight-megapixel RAW images on the Rebel XT model to 27 10.1-megapixel JPEGS and ten 10.1-megapixel RAW images on the new Rebel XTi camera. Also, with white balance bracketing enabled, up to 6 shots can be taken continuously, whereas only 2 shots were possible with the XT.
The camera starts up in a near instantaneous 0.2 second. Its shutter release lag time is only 100 milliseconds and viewfinder blackout time is a mere 175 milliseconds.
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