Here are some of the most frequently asked questions about sensor dust issues.
Q. I see tiny specks in my viewfinder. Do I have dust on my sensor?
A. If you see sharp, well-defined specks, they are clinging to the underside of your focus screen and not on your sensor. They have absolutely no effect on your photographs, and are merely annoying or distracting.
Q. I can see dust on my mirror. How can I remove it?
A. Like focus-screen dust, any artifacts that have settled on your mirror won't affect your photos. You can often remove dust on the mirror or focus screen with a bulb air blower, which will loosen it and whisk it away. Stubborn dust on the focus screen can sometimes be gently flicked away with a soft brush designed for cleaning lenses. I don't recommend brushing the mirror or touching it in any way. The mirror is a special front-surface-silvered optical device (unlike conventional mirrors, which are silvered on the back side of a piece of glass or plastic) and can be easily scratched. If you can't blow mirror dust off, it's best to just forget about it. You can't see it in the viewfinder, anyway.
Q. I see a bright spot in the same place in all of my photos. Is that sensor dust?
A. You've probably got either a "hot" pixel or one that is permanently "stuck" due to a defect in the sensor. A hot pixel is one that shows up as a bright spot only during long exposures as the sensor warms. A pixel stuck in the "on" position always appears in the image. Both show up as bright red, green, or blue pixels, usually surrounded by a small cluster of other improperly illuminated pixels, caused by the camera's interpolating the hot or stuck pixel into its surroundings, as shown in Figure 9.6. A stuck pixel can also be permanently dark. Either kind is likely to show up when they contrast with plain, evenly colored areas of your image.
Finding one or two hot or stuck pixels in your sensor is unfortunately fairly common. They can be "removed" by telling the 40D to ignore them through a simple process called pixel mapping. If the bad pixels become bothersome, Canon can remap your sensor's pixels with a quick trip to a service center.
Bad pixels can also show up on your camera's color LCD panel, but, unless they are abundant, the wisest course is to just ignore them.
Q. I see an irregular out-of-focus blob in the same place in my photos. Is that sensor dust?
A. Yes. Sensor contaminants can take the form of tiny spots, larger blobs, or even curvy lines if they are caused by minuscule fibers that have settled on the sensor. They'll appear out of focus because they aren't actually on the sensor
surface but, rather, a fraction of a millimeter above it on the filter that covers the sensor. The smaller the f/stop used, the more in-focus the dust becomes. At large apertures, it may not be visible at all.
Q. I never see any dust on my sensor. What's all the fuss about?
A. Those who never have dust problems with their EOS 40D fall into one of four categories: those for whom the camera's automatic dust removal features are working well; those who seldom change their lenses and have clean working habits that minimize the amount of dust that invades their cameras in the first place; those who simply don't notice the dust (often because they don't shoot many macro photos or other pictures using the small f/stops that makes dust evident in their images); and those who are very, very lucky.
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