Dust is an annoying fact of life for digital SLR users. However, Canon's newly announced EOS 400D (called the Digital Rebel XTi in the US and elsewhere) aims to combat dust with a multipronged approach, going further than competing digital SLRs do.

The EOS 400D is the first Canon SLR with the company's EOS Integrated Cleaning System. "It's really a comprehensive dust removal system, one that addresses the issue of how dust is generated as well as how to suppress it in images, both in hardware and software," says Chuck Westfall, director of media and customer relationship at Canon.

Dust particles tend to get into digital SLR cameras when you change lenses. How dust appears in an image -- be it an amorphous blob or a series of specks that clearly don't belong -- will vary from shot to shot, depending on the aperture you're using. The aperture affects the depth of focus, and the greater the depth of focus, the more readily the dust particles show up in the image.

Canon is not the first to offer in-camera dust-removal features. Olympus was first to market last year, with its vibrating dust-reduction system. Sony's recently introduced Alpha digital SLR has a mechanism that shakes dust from the sensor whenever you turn the camera on or off; Sony says the camera's imager also has special coating to deflect dust.

The EOS Digital Rebel XTi camera will be available for about £615 plus VAT with a zoom lens kit, while the body-only kit will carry an estimated selling price of £550 plus VAT.

 border=0 /><BR></div>
</p>
<p>
For starters, Canon paid attention to the camera

"With the old body cap, there was the possibility of shavings falling off with normal wear and tear," says Westfall. The company has already used this body cap on the EOS 5D.

The core portion of Canon's new technology, though, is the redesign of the camera's low-pass filter, whose primary function is antialiasing and the elimination of moiré, to give accurate color under all conditions.

Unlike previous designs, in which the low-pass filter is a single unit, "the low-pass filter on this camera is divided into two components, a front and a rear," says Westfall. "The front component, which is presumably where the dust would be adhering, is actually positioned further out in front of the CMOS sensor than it is on other cameras. The result is, the dust particles are less likely to appear in images because of the position of the front plate of the low-pass filter. The back plate of the filter is within a millimeter of the sensor; the front plate is an additional millimeter or two out in front. That distance puts the front surface further out of the depth of focus as you stop the lens down. This way, it is less likely to show any dust, because the filter is further away."

 border=0 /><BR></div>
</p>
<p>
Beyond the dust minimization techniques Canon has adopted, the company has followed Olympus and Sony in offering a self-cleaning image sensor as well, one that uses vibrations to shake dust particles off the sensor.
</p>
<p>
The 400D

"The intention is to shake any of the loose dust particles off of the low-pass filter," says Westfall. Particles are then deposited onto a strip of adhesive material, to prevent them from ending up back in the low-pass filter later. Says Westfall, "I use the analogy of the old Roach Motel commercial: Dust particles check in, but they don't check out."

 border=0 />In spite of the aforementioned precautions, dust in a digital SLR can be tricky -- and pesky -- business, and it

"Dust Delete Data makes it possible to generate a map of any residual dust on the low-pass filter," explains Westfall. Select the feature in the menu, and shoot an image of a white piece of paper (or a white wall or some other subject that has no discernible contrast). The camera takes a picture of the low-pass filter and creates an electronic dust map. "This Dust Delete Data is stored in the camera's memory, and the data is appended to subsequent images that are captured. It doesn't make any difference if the image is RAW or JPEG; the info is appended to the file."

After you download the image to the computer, the Dust Delete Data can be processed out of the image with Canon's Digital Photo Professional Software version 2.2, which comes with the camera. "The nature of most dust particles is that the detail is not gone, it's just darker or lighter," says Westfall. "The objective here is to blend in the density of the area that the dust particle is covering. We're not inventing subject detail that didn't exist, but we are altering the density of the area, so it blends in with the background."

Canon isn't the first company to provide a map of dust's location on an image. Nikon has offered a Dust Reference Image option on its digital SLRs for the past couple of years. However, Nikon's system requires you to shoot images in RAW format, and the Dust Reference Image data doesn't travel with every image, which makes it a pain to use. Plus, you need Nikon's Capture NX software -- a £102 option -- to remove dust using the Dust Reference Image.