Sensor Cleaning: Where Is That Dust Hiding, and What Do I Do About It?
Every image you shoot on a film SLR is made on a fresh frame of film. Even if somehow you do get dust on one, the next one is likely OK, and after 36 exposures you get to start over again with a new clean roll. This "paper towel dispenser" approach just doesn't carry over to digital though. Every image you ever shoot on a digital SLR will be made using the same CCD or CMOS sensor, and sooner or later you will have to deal with dust spots on that sensor.
Being overly cautious and probably more concerned with liability than practicality, most digital camera manufacturers don't endorse cleaning the sensor yourself. Especially for those who shoot outdoors where dust is unavoidable, learning how is almost a necessity.
Rather than trying to duplicate the excellent work of others who have already covered this topic extensively, I wanted to take this opportunity to point readers at some good sources and provide a few observations and tips of my own.
While he didn't actually invent the idea of cleaning your own sensor, no one has done more to popularize the idea than Nicholas R. His Copper Hill Images site on PBase has long been a reliable source for unbiased information on how to do it. Even though I know how to clean mine now (thanks, Nicholas), I check out his site periodically anyway as he is constantly updating it. The guy truly has a passion for figuring out the best way to do this. Other sites have copied much of his work but I'd rather trust someone who knows from first hand experience how best to do it and has been at it since the beginning. Experience matters. After pioneering the cut-down spatula ("Spat-U-Swab") to replace the anemic SensorSwab, he improved things further with the introduction last year of the SensorSwipe custom made tool. Carefully fold a PecPad over the end of a SensorSwipe and apply a drop or two of Eclipse fluid and you've got a mean, lean sensor cleaning machine. Nicholas now sells other cleaning supplies as well so you can get everything all from once source.
The latest potentially interesting method of sensor cleaning is the Sensor Brush by a Canadian company called VisibleDust. Looking like a high-tech artist's paintbrush, it is designed to attract dust via a static charge induced by spraying the end of the brush with canned air. It works well for loose dust, but can't handle the specs that somehow seem to get fused to the surface of the sensor occasionally. I also worry about how well it will hold up over time. Brushes are rigorously inspected and laboratory clean when brand new, but keeping it that way over time seems a challenging goal, but one that must be met if it is to keep doing its job well. That seems chancy and you are gambling with your sensor. Yes, you can wash it with mild soap, but this isn't practical after every cleaning so you will have to live with reusing your brush and hoping for the best. Keep your fingers off the brush so you don't get oils on it.
One truly odd idea floating around the internet is to use Scotch tape to clean your sensor. The story goes that if you stick it on and then pull it off, it will take all the dust with it when it goes. Not on my sensor at least. For some reason, I feel more comfortable with the time tested method. Anyone who's ever had gum residue left behind when removing tape should know enough to be wary of ideas such as this.
You probably already know if your CCD or CMOS sensor is suffering from dust spots, but if you want to see just how bad things actually are, stop the lens down to f/22 and shoot an evenly illuminated bright subject. A cloudless blue sky can work well, as can the surface of a light table if you still have one from your slide film days. I just set the lens face down on my light table and fire the shutter. Focus doesn't really matter. Open the resulting image in Photoshop and run an Auto Levels on it or otherwise exaggerate the contrast as much as possible. Even a perfectly clean sensor may show an odd pattern, but it's the spots you are looking for. Don't freak out if you see large splotches you never knew were there. Auto Levels has a way of making even slight color or contrast differences quite evident. It's the dust we're interested in.
Try to be methodical when you clean so you can minimize the time required. Work in a clean space as well so you don't end up making things worse. To better see what you are doing, get a headlamp. I use a Petzl one for hiking before sunrise or after sunset, so I now use it for sensor cleaning as well. I definitely recommend using the AC adapter and locking up the mirror too. Doing so is cheap insurance against having to replace the shutter if it closes on you while cleaning in Bulb mode.
When you can't or don't have a full compliment of cleaning supplies with you on a trip, a good blower brush such as Giotto's Rocket can help some. Be careful with blowers though as they have a tendency to just move the dust around if you're not careful. If you must use a blower, fasten your camera on a tripod so the body opening faces down to help make sure anything you dislodge will find its way out of your camera rather than landing elsewhere inside it. Even if you do have Eclipse and a swab though, look at your sensor and blow off any dust you can see. Most dust spots on your images will be caused by specs small enough to be invisible to the naked eye but make sure you don't have anything bigger to contend with before you wipe. Or brush for that matter if you choose to go the Sensor Brush route.
If you clean the sensor and find you still have spots that resist your normal routine, don't despair, but don't resort to rubbing either. Applying excessive pressure is a sure way to increase your odds of damaging your sensor. By and large, cleaning it is quite safe, but if you work at it with too much vigor, you can cause yourself big problems.
To understand just where stubborn spots are, we'll need to consider for a moment how lenses work. Take any lens and remove both caps (shorter focal length lenses will work better). Hold it up to the light and look through it. Hold your other hand out in front of the lens and move it around. You will find that a lens reverses the image both left to right and top to bottom. But as soon as you turn your point of view around and look down into the camera, you will have reversed things once again left to right. In the end, the image projected on the sensor is reversed top to bottom, but not left to right since the two side to side reversals cancel each other out.
You are generally better off cleaning the entire surface as normal though since cleaning only part of the surface may leave a streak at the edge of where you clean. Sometimes giving up until the next day can be the answer. For whatever reason, stubborn spots one day can often be much more cooperative if you let them rest a while between cleaning attempts. Don't go overboard though. If spots still persist, get the sensor cleaned professionally.
Dust is a fact of life with digital SLR cameras. Once you get over the initial fear of cleaning your own sensor though, it's really not that bad.
Update 2/07/2005 - A reader from the UK reminded me I should have mentioned that Olympus E-series digital cameras feature a patented ultrasonic dust removal system that causes most dust to be vibrated right off the sensor. Where it ends up is a mystery, but at least temporarily it won't be on the CCD. I have no doubt that other innovative technologies will emerge in the coming years to help with the dust problem, but for now, self-cleaning is the answer for many of us.
Update 6/12/2005 - Copper Hill now offers a brush called the "Sensor Sweep" for those who prefer this option. So long as you keep it spotlessly clean, it can work for dust not "fused" to the sensor surface.