Understanding
Digital SLR Sensor Cleaning

Our Dirty Little Secret

There comes a day when every DSLR owner looks at a 100% magnification of one of their images in Photoshop and gasps, "Where did that dust come from?" Nasty out of focus blobs. Squiggly little dust motes. Hairs. Uggh.

As scary as it may seem there is nothing for it but to clean the camera's sensor. It really isn't that tough to do, but you do need the right tools and approach. Here is the method and the supplies that I use. Work slowly. Work carefully. Use the right supplies and you should have no problem.

Checking for Sensor Dust

Checking to see whether your sensor needs cleaning is simple enough. The following method works indoors at night, where you will most likely be when cleaning your image sensor. Simply attach a relatively long lens to the camera, focus the lens manually at infinity, stop it down to its smallest aperture, set the exposure compensation to +1 stop, and photograph any nearby solid white or light colored surface. A laptop screen displaying an all white full screen image works very well, but even a piece of paper or a wall that is reasonably well illuminated will do. The surface being photographed should be no more than a few feet away. The idea is to make the item being photographed extremely out of focus so the only details in the final image will be dust spots and not features of the item being photographed. The small aperture makes the light rays between the back of the lens and image sensor parallel to one another and perpendicular to the glass sensor cover. That causes any dust on the glass cover to cast more distinct shadows on the image sensor. The long focal length makes it possible to assure the image is completely out of focus at the lens's smallest aperture, which would be impossible with a very wide angle lens.  The long focal length also makes it possible to use something small and evenly illuminated as a test subject. After this is done, simply view the test image on a computer at 100% (actual pixels) magnification. What you see is the maximum possible amount of dust you will see in any normally processed image. It is a judgment call as to whether it is enough to warrant cleaning the image sensor. Some use "Auto Levels" in Photoshop to make dust spots stand out even more. I do not do this when determining whether to clean the sensor because the extreme contrast boost shows spots that would be invisible in any normally processed image. Auto levels can be a good tool to use after you have decided to do a thorough solvent cleaning, but it is not necessary. If I am in the field shooting wildlife I sometimes even make an additional exposure as described above, but at a wider aperture like f/8 or f/11. Since wildlife photographs are typically taken using large apertures, this additional test image gives a more realistic idea of whether spots I see in the first test image may be showing up in the my photos. The smallest spots visible in a test image made at, say, f/32 will be invisible at f/8 or f/11. Likewise, many of those visible at f/8 or f/11 vanish at wider apertures. 

The principal by which static-chargeable brushes work is simple. Certain materials, like nylon or hair, have a tendency to accumulate a static electrical charge when rubbed against certain other materials, like paper or even air, at a reasonably high speed. The bristles of sensor cleaning brushes are made of such a material. To charge the brush you blow air through it or swipe it back and forth a few times over an appropriate and clean surface. Once charged, the brush is very lightly passed over the surface of the image sensor in your camera. The static charge in the bristles makes any dust particles encountered stick to the brush instead of just being pushed around by it. Brushes with static-prone bristles and insulating handles have been around for thousands of years. The only thing new about any of this is using one to clean a digital camera's image sensor. It is quite possible to source an appropriate brush yourself. They are amazingly cheap, but buying and testing them to find the right ones would be a time consuming hassle. For more detail I recommend this excellent myth-busting article about sensor cleaning brushes.

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Back in May of 2004 a company called Visible Dust introduced a line static-chargeable sensor cleaning brushes. These were priced at $34.95 until sometime in November 2004 when the price skyrocketed to about $100 US. At the time Visible Dust used photos of plasma chambers along with talk of DNA and biochip technology to sell what was simply a wooden handled brush at insane prices. Today they have more products than one can imagine, including motorized twirling sensor brushes, outrageously priced solvents, lighted sensor loupes, and at least one very expensive brush for each unique part of your camera. These folks are really cleaning up (pun intended) on the hype and paranoia that exists over cleaning cameras. While many like the Visible Dust products, the only static-chargeable brush I can recommend is the high quality SensorSweep brush currently sold by Copper Hill Images for a sane price of $22.95 plus shipping and handling. They ship quickly and the brush comes with a dust-tight carrying tube and instructions. The brush works well, but shares all the problems any brush will obviously have. That said, static-chargeable brushes are another option that under certain conditions can be very handy to have in your arsenal of sensor cleaning tools. I personally use the brush only after wet cleaning, to clean up the few small dust specks that may be left along edges or in corners of the image sensor.

Note that Photographic Solutions warns that its Pec* Pads are not meant for cleaning image sensors, and that they provide no warranty if Pec* Pads used for this purpose cause damage. They also say that Pec* Pads are safe to use on film emulsion, which is quite soft and scratch prone, mirrors, telescopes, and lenses. The obvious question is why something that will not damage film emulsion, mirrors, telescopes, and lenses might damage the filter over the image sensor in your camera. Fortunately, David Stone of Photographic Solutions took some of his time to explain this to me, and it boils down to two basic facts.
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1. The safety of Pec* Pads has not been tested with respect to image sensors, but it has been tested with respect to film emulsions.

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2. Cleaning a camera's image sensor requires much more handling of the Pec* Pad than it requires handling of a Sensor Swab, so the risk of contaminating the Pec* Pad with abrasive dirt or oily fingers is greater. Since Photographic Solutions has no control over how the Pec* Pads are used, they discourage their use for cleaning image sensors.

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That's fair enough. Because these products are used under all sorts of circumstances by people with various levels of understanding and skill, I would personally have a hard time guaranteeing the use of either one. I simply prefer to assume the risk and save the money. Sensor Swabs are nearly $4.00 US each, while Pec* Pads are only $0.07 each, and it often takes more than one per cleaning. If you only need to infrequently clean the image sensor on one camera, and doing it stresses you out, the Sensor Swabs may make more sense. The choice is yours.

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To stay out of trouble using the Pec* Pads we always:

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Conclusion

I have tried to write something here that might help those trying to clean their image sensors without going bankrupt. When it comes to sensor cleaning it turns out that the least expensive methods are also among the most effective. How refreshing! In any event, I hope this article helps you maintain spot-free digital image captures. If not, you can always fall back on the clone tool. 

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Happy sensor cleaning,