A film SLR pulls a fresh section of film out of the cartridge before taking every picture, but a DSLR's sensor remains stationary. This means that if a dust speck lands on the low-pass filter in front of the sensor, it will stay in place, making a black mark on every picture until you clean it off (Figure 2.2).
1 APS-C originally denoted the "classic" picture format on Advanced Photo System (APS, 24-mm) film, and APS-H was its "high-quality" format. "Four Thirds" denotes an image size originally used on video camera tubes that were nominally 4/3 of an inch in diameter. Many video CCD sensors are still specified in terms of old video camera tube sizes, measured in fractional inches.
Figure 2.2. Effect of dust (enlarged). Diagram shows how light gets around the dust speck so that stars are visible through the blotch.
To keep dust off the sensor, avoid unnecessary lens changes; hold the camera body facing downward when you have the lens off; never leave the body sitting around with no lens or cap on it; and never change lenses in dusty surroundings. But even if you never remove the lens, there will eventually be some dust generated by mechanical wear of the camera's internal components.
When dust gets on the sensor (and eventually it will), follow the instructions in your camera's instruction manual. The gentlest way to remove dust is to open the shutter (on "bulb," the time-exposure setting) and apply compressed air from a rubber bulb (not a can of compressed gas, which might emit liquid or high-speed particles).
If you must wipe the sensor, use a Sensor Swab, made by Photographic Solutions, Inc. (www.photosol.com). And above all, follow instructions to make sure the shutter stays open while you're doing the cleaning.
Some newer DSLRs can vibrate the sensor to shake dust loose. This feature was introduced by Olympus and is included on the Canon Digital Rebel XTi (400D).
To a considerable extent, the effect of dust can be removed by image processing, either flat-fielding (p. 185) or "dust mapping" performed by the software that comes with the camera.
You can check for dust by aiming the camera at the plain blue daytime sky and taking a picture at f /22. Note that dust that you see in the viewfinder is not on the sensor and will not show up in the picture; it's on the focusing screen. Removing it is a good idea so that it does not eventually make its way to the sensor.
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