Framed and Exposed: Out, Damn Spot!
If you've spent much time scanning film, you know that managing dust and scratches can be a hassle. You'd think that shooting digital would free you from this dusty chore, but not if you use a digital SLR. Because you can remove the lens, dust can worm its way inside the camera body, around the mirror, and stick to the sensor. Once there, the dust can show up in your images as dark specks or smudges.
Figure 1. This image suffers from specks and smudges caused by dust on the camera's sensor.
Some SLRs seem to be more prone to dust than others. I've heard speculation that it's because certain sensors require more of an electrical charge than others, which creates more static electricity on the sensor's surface, which attracts more dust. Other cameras take an offensive position: The Olympus Evolt E300, for example, automatically shakes its sensor every time you power up the camera in an effort to dislodge any dust or particulate matter that may have landed on the sensor surface.
You might never encounter a dust problem. But if you do, there are several ways to eradicate it. The best way to deal with dust, though, is to avoid it in the first place.
The easiest way to keep dust from getting into your SLR is to never remove the lens -- but that's not exactly practical. Instead, you can be smarter about when and how you change lenses.
When you know you'll be shooting in a windy, dusty, or sandy location, plan ahead by mounting your best general-purpose lens on your camera. You might be able to avoid a lens switch while you're in the danger zone. If you do have to change, move to a relatively sheltered location.
In more normal situations, there are simple practices that will keep dust hazards to a minimum. Before removing your lens, have the replacement lens ready for a quick swap. Point the front of the camera down while changing lenses so dust can't fall into it from above. To pull this off, you usually need one hand to hold the camera, with the replacement lens close by.
Keeping your camera clean is also smart, since dust and debris on the surface can easily fall inside the body when swapping lenses. Inside your SLR, there's a mirror that bounces the light from the lens up into the viewfinder, and this mirror offers a good amount of dust protection. The mirror/sensor chamber can accumulate dust, though, so keep this area clean to prevent dust from slipping onto the sensor.
The most important thing to know about cleaning your camera is that you should never, ever, at no time, use canned, compressed air to blow out the inside of your camera.
This is very important, so I'm going to copy that last sentence and paste it here: The most important thing to know about cleaning your camera is that you should never, ever, at no time, use canned, compressed air to blow out the inside of your camera.
The propellant in compressed-air cans sometimes sprays out a liquid that quickly evaporates but leaves a residue on your sensor surface. Not the result you were hoping for.
With all compressed-air cans far from reach, take a different strategy to clean out the chamber of your SLR. A blower brush, such as you can find at any camera store, is one tool for cleaning off the surfaces of the mirror chamber. The blower part is more useful for this than the brush part. The hairs of the brush can snag on things inside the camera. So, don't brush around inside the camera -- just use the blower. Keep the camera tilted down while blowing.
Most blower brushes are small and don't put out a lot of air. A much more effective tool (though one that's much more embarrassing when it falls out of your camera bag at inopportune moments) is a rectal syringe. These things put out a lot more air than the typical blower brush. (With great restraint, I am avoiding puns about "backing up," "stuck pixels," or "aperture size," but it isn't easy.)
Figure 2. A rectal syringe is a more effective camera cleaner than the typical blower brush at a camera store.
Cleaning your camera and lenses once or twice a week will greatly reduce your chances of developing sensor dust problems.
What Dust Looks Like
No matter how hard you try, you may still end up with a dust problem. If this happens, consider cleaning your camera's image sensor, a topic I'll discuss in the next Framed and Exposed. However, if you don't have the proper supplies, are in the middle of a shoot, or are wary of the potential dangers of cleaning your own gear, try working around your dust problem.
No matter how bad your sensor dust is, you can probably shoot around it by sticking with wide apertures. Below are two images shot moments apart using the same camera and lens. As you can see in the first image, shot at f/20, this camera is suffering from dust.
Figure 3. I shot these images with different apertures. The left image was shot with a small, dust-revealing aperture of f/20, while the cleaner image on the right was shot at f/5.6.
The second image was shot at f/5.6 and, though the dust problems are still faintly visible, they're much less pronounced. At a wider aperture, your camera's depth of field is so reduced that it can focus past the dust to shoot a cleaner image.
Of course, a reduced depth of field is inappropriate for some images, and your lens' wider apertures might not yield the most sharpness, but these sacrifices can be worth it if you must get a clean shot.
You may also find that dust spots are hidden by your subject matter. A busy foreground, for example, may conceal a lot of trouble spots, while skies and fields of color reveal even minor dust smudges.
Alternately, you can shoot as you normally would and digitally remove the dust later in an image editor. Most dust spots are easy to banish using standard clone and rubber stamp tools.
The ideal way to handle dust is to get it off your sensor. In the next installment of this column, I'll discuss several methods for safely cleaning your digital SLR's image sensor.
Read more by Ben Long.
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