Understanding Dust

Dust is a fact of life in photography, whether digital or analog. Dust spots negatives, gets into slide scans, makes its way onto and into lenses and viewfinders, and ends up on sensors. Dust is everywhere. Unless you do all your photography in a microprocessor fabrication plant, it's impossible to get rid of it. The best you can do is manage it. To be able to do this effectively, it helps to understand a few things about what dust is and how it behaves.

Dust comes in a tremendous variety of particle sizes, from clusters of a handful of molecules to fluffy bunnies that float lazily in the sunlight. We're most concerned with medium-sized dust: particles big enough to be seen if they land on the sensor, but small enough that surface adhesion will keep them there when you blow air on them.

Unfortunately, this size dust is still very, very small -- far too small to be seen with the naked eye, and small enough to happily make its way through joints and cracks in the lens, camera, and shutter curtains, especially as focusing and zooming pumps air in and out of the box. Yes, even through weather seals.

Dust levels within connected areas will equalize over time. That means that since your camera isn't sealed and air is going in and out of it, it's only a matter of time -- and not a long time at that -- before the dust concentration inside is the same as the dust concentration outside. Only the largest particles will get stopped by the seals and joints in the camera, not the ones that make those little smudges on the sensor.

Dust floats. Dust is so small that you can almost ignore gravity when considering how it behaves. It floats on the air just like plankton floats in water. Yes, in a still room it will eventually settle down, but this happens very slowly. The visible dust that gathers on furniture (both kinds, the light kind that collects on dark furniture and the dark kind that collects on light furniture) is much larger than the kind of dust we're interested in, and settles much more quickly. Even so, it'll take the better part of a week for that kind of dust to produce a layer on a surface you've just wiped.

Dust clings. When a dust particle touches a surface, it will stick to it. This is known as surface adhesion. If electrostatic charges are involved, dust will actually get sucked to a surface from some distance. However, I believe that electrostatic effects are of very minor importance when considering camera sensors (and cleaning them). Unlike many people believe, the imaging sensor is not "charged" most of the time, and when it is charged, the charge is tiny; too weak to attrack dust further than a couple of millimeters. Similarly, I believe that the VisibleDust theory that dust sticks to their brush through electrostatic attraction is false -- I'm pretty sure it's a matter of simple surface effects.

The biggest source of dust is... you. We people are constantly shedding dust -- dust we've collected outside, dead cells, dead hairs, dead mites, live mites, fibers from clothes we're wearing, and so on.

What does this mean in practice?

• Don't sweat about changing lenses. Most of the dust inside your camera would have made it in even if you never changed a lens. And it really doesn't matter which way up you hold your camera: the time the lens is off the camera is much too short for dust to have time to float down with gravity.
• Don't get too anal about eradicating dust from your cleaning area. Since dust levels tend to equalize, even if you do your cleaning in a perfectly dust-free environment, it won't take long for dust to make its way right back in. Cleanliness is important, but it's enough that your cleaning area is significantly less dusty than your regular shooting environment. In particular, try to pick an area as free of mineral grit as possible: for example, don't clean in a room with a vent that opens directly to a busy city street.
• Don't get too close to the camera when cleaning it and if there's an air current, keep downwind of the camera.
• Dust must be seduced, not forced. To get dust off a surface, your best bet is to provide it with another surface it likes better. Dust isn't particularly fond of very smooth, shiny surfaces. Fortunately, your sensor is extremely smooth and shiny. Therefore, most dust will be happy to abandon it if you present it with a surface that's less smooth. Such as a bunch of very thin, soft fibers on a brush.
• Find a way to get the dust off your brush. Everything in nature tends to equilibrium; so too your cleaning brush. If you do nothing, it will eventually accumulate so much dust that for every particle lifted on, one drops off.