Background

Dust happens, and anyone who uses a digital SLR camera will eventually need to clean their camera's image sensor. You should be aware that when we talk about cleaning the image sensor we are actually talking about cleaning a filter that covers the electronic CMOS or CCD element, which is the thing that actually produces your images. There is legitimate concern about scratching this filter, since any mark on it will show up on every image captured by the camera, but the dangers are often overstated. The best chance of scratching it happens when a piece of hard grit gets trapped in a cleaning tool and is wiped across the filter surface. This is why it is important to keep all cleaning tools clean and blow any loose particles off of the sensor with air before cleaning by any method that uses wiping.

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Not All Spots are Created Equal

Most spots that are visible on digital images are simply dust of one kind or another that floated into the camera body when changing lenses, were carried in on the back of the lenses, or were produced inside the camera itself. Some of this dust is sticky, like pollen or oily soot particles from diesel exhaust. In earlier Canon camera bodies tiny droplets of excess lubricant sometimes find their way to the sensor's filter. Some spots are liquid, or deposits left by evaporated liquid, produced by very tiny droplets like those that make fog. Still more dust is produced by the camera as its internal mechanical systems wear. Most dust can be blown off of the sensor with air from a hand or foot powered blower. Some cannot. One thing all these spot causing contaminants have in common is that their particles are very small. In most cases they are too small to readily see with the unaided eye. A typical digital SLR image sensor has in the neighborhood of 3500 pixels or more per linear inch, so it does not take a large particle to block light from a few pixels. This is also why even very small scratch on the sensor cover glass is something to be avoided. 

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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.   

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Cleaning with Blowers 

Blowing the sensor off with a hand or foot powered blower is the safest method, but it can remove only loose dust that is not stuck to the sensor. This cleaning method works well enough to get us by for a couple of weeks on location, but it does not do a thorough job of cleaning. In addition to not removing any debris that are stuck to the image sensor, it can add or simply move dust around on the sensor unless you do it carefully. It is best to mount the camera on a tripod with the lens opening facing directly downward so particles tend to fall away from the sensor and out of the camera when the sensor is blown off. This position is somewhat awkward, but one gets used to it quickly enough. This method never produces a completely dust -free sensor, but a few small specks on an images are no big deal. Depending on the shooting aperture, they may be completely invisible. In the worst case, cloning them out in Photoshop is quite easy.

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Cleaning with Brushes

Recently, "special" static-chargeable brushes have become available for image sensor cleaning. Using a static-chargeable brush to clean your camera's image sensor has potential advantages when it comes to air travel because they require no flammable solvents and are used dry. They can remove more debris from the sensor than would be possible with a blower, and if there are any hard mineral particles on the sensor a brush may be less likely to cause scratches than methods that involve wiping with a solvent. However, static-chargeable brushes will not remove debris that are stuck to the glass sensor cover. Only solvent-based cleaning will do that. More significantly, I have had small spots produce bigger streaks when a brush is used. I assume this happens only when the composition of the spots is somewhat gummy and the substance smears slightly when the brush bristle tips pass over it. The smear or streak is not huge, but it is bigger than the spot that produced it, and it is therefore more likely to show up in images. If this happens when there is no way to clean the sensor with solvent, you are worse off than you were before cleaning the sensor. While this situation does not always occur, you never know when it will. Because of this I hesitate to use static-chargeable brushes unless I also have access to solvent-based cleaning tools. But, if I have access to solvent-based cleaning tools I don't need the brushes in the first place. For me this negates the air travel advantages of owning the brushes. In addition, moisture in the air is a good conductor of electricity and it tends to let electrical charges neutralize themselves quickly. I would expect any static-chargeable brush to perform better in dry environments that it would in humid environments. Air conditioning can correct humidity problems to some extent, but if we were based in primitive accommodations in a tropical rainforest, as we sometimes are, I think it would be impossible to charge the brushes. Using these brushes to clean a camera when camping in damp or rainy conditions would pose similar problems.

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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.

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Cleaning with Solvents

In my opinion the very best way to clean an image sensor is by a method informally known as the Copper Hill Method. It removes every spot, of every type. It is also cheap and relatively easy once you learn the technique, though learning it takes a little practice. We have cleaned all of our image sensors this way countless times over approximately five years and it has never caused even the most minor problem. The Copper Hill Method uses a simple tool called the SensorSwipe. You can buy the tool from Copper Hill Images for just a few dollars. You can even make one of these yourself by cutting down one of the smallest spatulas you can find at Wal-Mart. But, it's far easier to buy one from Copper Hill. I have personally done both. Along with the Sensor Swipe tool you will need a bottle of Eclipse or Eclipse E2 solvent and some Pec* Pads, both of which are produced by Photographic Solutions and can be purchased at Adorama. The Pec* Pad is wrapped around the Sensor Swipe, a couple drops of Eclipse or Eclipse E2 solvent are put on the Pec* Pad, and the tool with the Pec* Pad attached is used to clean the camera's image sensor. One of the two Copper Hill web sites contains incredibly detailed and complete tutorials about how this and more are best accomplished. The Copper Hill sensor cleaning method has worked somewhat better for us than the much more expensive Sensor Swab, which is also produced by Photographic Solutions. This seems to be because the wiping surface is more evenly and firmly supported across its entire width by the Copper Hill Sensor Swipe. The Copper Hill Method also works better than anyone's brush or blower. The only problem I have ever found regarding this cleaning method is that you cannot transport the required solvents on commercial airlines because the Eclipse products are simply different types of alcohol, and are therefore quite flammable. Photographic Solutions Eclipse (or Eclipse E2) optical glass cleaner is key to this process because it leaves absolutely no residue and does not streak. A list of which cameras require which solvent can be found here.

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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,

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Notes:

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In recent Canon and other camera bodies, the sensor cover glass is coated with tin-oxide. This tin-oxide coating forms the sensor's infrared filter. In addition, tin-oxide is electrically conductive. Canon moved this to the front surface to better dissipate the static charge that can make dust stick to the surface. This may help their new vibrating sensor dust removal gizmo, which is also a recent development, work a little better. This vibrating gizmo is part of all camera bodies with the tin-oxide coated sensor cover, except the 5D. This is probably no coincidence, even though some reports indicate that Canon's self-cleaning image sensors do not self clean very well.
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There have been reports, or at least rumors, of damage when the tin-oxide coated filter surface is cleaned with "regular" Eclipse fluid, which is simply pure methyl alcohol. Photographic Solutions has developed a different cleaning fluid for these cameras called "E2". Based on the Material Safety Data Sheet (MSDS) for E2, it is composed of ethyl alcohol, methyl alcohol, and isopropyl alcohol. I have no idea why or how regular Eclipse (methyl alcohol) could damage a hard tin-oxide coating if the coating is properly bonded to the glass. Further, if methyl alcohol does damage the coating, I have no idea why a mixture of ethyl, methyl, and isopropyl alcohols would not also damage it. I suspect any problems must be few and far between, since many were cleaning Canon EOS 5D sensors with regular Eclipse long before E2 existed. With all of this said, and without knowing why it is needed, I would still recommend E2 for cleaning tin-oxide coated sensor cover glass. It certainly couldn't hurt, and it may help. A list of cameras for which the new E2 solvent is recommended can be found here. Note that while this chart used to recommend E2 for the Canon EOS 5D, it now recommends regular Eclipse for this camera. Based on this posting at dpreview.com by David Stone, President of Photographic Solutions, Inc., apparently there is no easy answer regarding which product is best to use on the 5D. "When in doubt, use E2" would seem to be the best rule to follow. An inquiry to Photographic Solutions regarding which solution is correct for cleaning the 5D produced the response "The coating on the 5D is on the bottom of the glass covering the sensor, therefore Eclipse works fine on it". Perhaps problem reports about using Eclipse on the 5D are false, or perhaps some sensor glass covers were installed backwards, with the tin-oxide coating facing out. Your guess is as good as mine. If I owned a 5D I think I'd still use E2 on it. Also, it seems that warnings against using E2 on glass filters and lenses is has now been removed from the labeling on newer bottles of E2, which makes perfect sense.

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As mentioned above, Canon is equipping all new digital SLR cameras with the vibrating dust removal gizmo, otherwise known as Canon's self-cleaning sensor. Frankly, sensor dust isn't enough of a problem to justify the complexity or expense of this contraption. But, because everyone freaks out over the sensor dust issue, Canon and others offer self-cleaning sensors as a marketing tool even though they apparently work poorly. To see this for yourself, check out http://pixinfo.com/en/articles/ccd-dust-removal/ and http://members.aol.com/bhilton665/tests/dust_400d/. Because these vibrating gizmos cannot remove all of the dust  that gets on them, Canon has also developed software to remove spots created by dust that remains on the sensor. I see this as a rather lame method in principal. One has to take a picture of a plain white surface with the lens focused at infinity to create a dust map so the software knows where the dust is located. But, every time you turn the camera on or off, the vibrating gizmo does it's thing, which can move some dust particles and make the dust map inaccurate from the start. Also, any dust that finds its way to the sensor after the dust map is made will not be part of the software's "map". Obviously, a single dust map image cannot work for processing a couple weeks worth of images from a remote location after one arrives home. It will remove some spots, but not all. Further, the size of dust spots in images varies with the aperture used to shoot a given image. Using one test image to remove dust from images shot at varying apertures is a compromise at best. All of this sounds like a lot more hassle than simply cleaning the sensor! This is especially true given that the vibrating gizmo combined with the dust removing software does not work well enough to eliminate the need for manual sensor cleaning anyway.
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If you spend lots of time and money on image sensor cleaning and paraphernalia, you would probably be far better off using your resources to make images instead. Sensor cleaning is far easier than all the hype would have you believe. A good cleaning kit costs only a few dollars and will keep your images as spot-free as any.