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Extraordinary New Dust

Reduction System

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 suppress-

ing red ghosting and color casts caused by reflections on the sensor surface. The sub-

ject image divided into two images horizontally by the low-pass filter is separated verti-

cally 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 fil-

ter #2 also serves as the CMOS sensor package's cover glass, eliminating a relatively

expensive glass cover and reducing cost.

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 liq-

uid 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 sen-

sor 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 over-

all strategy is one of, first, neither generating nor attracting dust. The shutter unit (the

same as in the Digital Rebel XT) generates minimal dust; the body cap (incorporated

since the first half of 2005) 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 notice-

able 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. Low-

pass filter #1 on the front of the sensor is attached to an ultrasonic

vibrating unit driven by a piezoelectric element. When LPF #1 is subject-

ed to ultrasonic vibrations, the adhering dust is shaken off the surface.

III. FEATURE DISCUSSION

Piezoelectric element

Vibration

LPF-1

Dust

Retaining

component