This page is designed to help the countless new DSLR owners who discover that their new camera is afflicted with that dreaded scourge of the DSLR shooter:

Sensor Dust

Below we see a typical example of a DSLR image with dust spots.

Sensor Dust example 001

The arrows point to the most blatant spots in the image.  There are many smaller ones as well.

This photo was taken with my Canon 20D within a month of receiving the camera.  I had only one lens and had not removed it from the body the whole time.  So I was dismayed to see these spots appear in my images.  I wondered if the camera was defective because I had not heard of "sensor dust".

By doing a bit of Internet research I found that these spots are quite common and are the result of tiny pieces of dust which have settled onto the camera sensor's filter.

Such dust casts shadows onto the sensor as you take your photos and thus shows up as darker spots, smudges, or squiggles depending on the size and shape of the dust particle.

Smaller lens aperture settings (higher f/numbers) exacerbate the problem because the smaller aperture appears as more of a point-source for the light entering the camera and thus causes the dust particles to cast more distinct shadows.  Think of the way you cast a distinct shadow on a clear day versus the way you may cast no visible shadow at all on a cloudy day when the light comes from the entire sky instead of the "point source" of the sun.

Some people never notice that they have dust on their sensor filter because they shoot using large lens openings (small f/numbers).  Some people go so far as to recommend that you avoid using small apertures in order to keep dust from being visible in your shots!  Personally, I feel that you should be able to use any setting your camera has and the choice of f/stop setting should be based on other considerations.

Where is this darn dust, how do I get rid of it, and how do I prevent it from happening in the first place?

DSLR Dust Rules:

Dust that shows up in your photographs pretty much has to be on your sensor filter.

Dust that shows up in your viewfinder pretty much has to be on your focusing screen.

Focusing screen dust cannot show up on your photos.

Sensor filter dust cannot show up in the viewfinder.

What about dust on the mirror?

Dust on the mirror cannot show up in your photographs because the mirror flips up out of the way when you take a picture.  And dust on the mirror will not be clearly visible even in the viewfinder because it will not be in focus.  If you've got a major-league dust bunny or hairball on the mirror, you might notice an indistinct shadow through the viewfinder, but it would have to be a large piece for that to happen.

In general, dust on the mirror won't cause any problems at all.  The mirror has a delicate surface that is easily damaged, and since the mirror is mounted in a delicate manner and its position is very critical to both autofocus (due to the relay mirror mounted to it) and manual focus (because its position affects the light path length to the focusing screen), I highly recommend that you treat the mirror with extreme care.

How about dust on the front or back of the lens?

As with dust on the mirror, dust on the lens will generally not be "in focus" at the sensor or the focusing screen.  Dust on the lens surfaces will lower contrast by scattering light and can cause extra flare or a general lack of sharpness in an image.  But again, it normally won't cast any distinct shadows and so can't show up in your photos.

Huge chunks of dirt or raindrops on the front element (or the front surface of a filter used on) a wide angle lens may show up in photos taken at small apertures.  The depth of field for such an arrangement can be so great that large things on the lens can be visible.

But tiny pieces of dust won't show up as spots even with a wide angle lens.

Here is a shot where you can see raindrops on the front filter of a Canon 10-22mm lens used on a 20D.  It's time to take the camera back inside, I guess.

Visible raindrops in photo taken with a very wide angle lens.  The drops were on the front filter of the lens.

Of course, raindrops are large and easy to see so you'd know you needed to clean the front of the lens in this situation.

It pays to keep your lenses clean, but generally, when you see spots in the viewfinder or on your photos, the problem will NOT be dust on your lens.

DSLR camera design:

A DSLR's construction is just like a "normal" film SLR in many ways.

Cross-Section of typical DSLR

Studying this drawing, you can see that when viewing through the DSLR, the light enters through the lens and strikes the primary mirror.  This mirror is semi-silvered and reflects much of the light upward.  That light then focuses and forms a visible image on the matte surface of the focusing screen.

The focusing screen behaves as a rear projection screen.  This is similar to the screens used in LCD or DLP Projection type television sets.  The image is projected onto the back side and we view through the clear plastic to see this image from the other side.  In the case of the SLR camera, a pentaprism or pentamirror arrangement allows us to see the image from the convenient eyepiece on the back of the camera.  This allows for the familiar and comfortable "eye level" camera SLR position when composing and shooting.

The eyepiece lens serves to make the projected image appear to be approximately 1 meter from the photographer's eye.  That "apparent viewing distance" is chosen to make it easy for most people to see the focusing screen clearly with their normal eye correction (glasses, contacts, or unaided eye).  Basically, this is a comfortable viewing distance for most people.

Troubleshooting Camera Dust:

From this drawing, you can see why dust on the mirror would not form a focused and distinct image on the focusing screen.  So dust on the mirror is not likely to be visible through the viewfinder.  Thus the rule that dust on the mirror isn't likely to be visible in the viewfinder unless it's huge!

You can also see how dust on either the mirror or the focusing screen cannot affect the photos taken because the mirror flips up to the position shown at number 3 during the exposure.  This configuration allows the light from the lens to pass through the "mirror box" unimpeded and focus directly onto the sensor.  Thus the rule that dust visible in your photos cannot be on the mirror or the focusing screen.

It should also be clear from this drawing that dust on the lens surfaces would not form clearly defined shadows either onto the focusing screen when viewing through the eyepiece or on the sensor when the exposure is actually being made.  Dust on lens surfaces will generally not be in good focus at the focusing screen or sensor.  Thus the general guideline that if you see dust through the viewfinder or see dust shadows on your pictures, the lens is unlikely to be the cause.

But, we can also see from this drawing that dust on the focusing screen will show up very clearly when viewing through the camera's viewfinder.  It can't affect our photos, of course, but it may be annoying.  Thus the rule that if you see dust through the viewfinder, the first thing to suspect is the focusing screen.

And we can also see that dust on the filter in front of the sensor can cast shadows onto the sensor during the exposure of an image, and thus show up in our photographs.  Once again, dust on the sensor filter cannot show up in the viewfinder.  Thus the rule that if you see dust spots on your photographs, there must be dust on the sensor's filter.

So now we've established where the dust is.  What now?

Mirror Dust:

Dust on the primary mirror will not be distinctly visible through the viewfinder and will also not show up in your photos.

The primary mirror is semi-silvered to allow some light to pass through it so that the autofocus system can get its "view" through the lens via the relay mirror.  The position at which the primary mirror rests is critical to the functioning of the autofous system.  Likewise, the position at which the attached relay mirror rests is also critical to the correct operation of the autofocus system.  If you knock either mirror out of alignment somehow, your AF will be out of calibration.

The semi-silvered surface of the primary mirror is also delicate and could be scratched quite easily.

Since dust on the mirror won't be visible on your photos, and since only huge chunks of dust or lint would even be detectable when viewing through the eyepiece, there is little to gain and much to lose by attempting to clean the mirror.

Still, there could be times when you find it necessary.  In that case, be very gentle so as not to disturb the mechanical alignment of the mirror(s).  And be careful of the surface so as not to scratch it.  I recommend that you simply use a bulb blower for this too.  If it works, then you have not touched anything at all.  You gain access to the mirror simply by removing the lens.  DO NOT use the sensor clean mode!  The sensor clean mode flips the mirror up so you would not be able to get to it's front!

Mirror Photo

The red arrow indicates the primary mirror.  Simply removing the lens reveals the mirror.

A bit of exhaled breath into the mirror box can provide moisture to dissipate static that might be holding stubborn dust to the mirror.  So you could give it a nice, easy breath and then blow it off with your bulb blower.

Now please understand this:

When I say to breathe into the camera's mirror box, I'm not asking you to blow into it!  Blowing with your lungs and mouth is often something that comes to mind when we want to blow dust or lint off of something.  But the problem is that if you blow hard enough to do any good, the velocity of the air coming out of your lungs and through your mouth will be high enough to pick up droplets of spittle.  You don't want to blow droplets of spittle into your camera or onto a lens, etc., so don't blow!  Yeah, you can clean it off later, but it's much better to just not have it happen to begin with.

So when I suggest breathing into the camera to deposit some moisture or humidity, I'm asking you to breathe out very slowly with your mouth wide open so that you do not generate any high-velocity air but instead just get a nice, slow, gentle flow of moist air from your lungs to go into the camera.  In fact, by exhaling very slowly, you may actually increase the humidity of the air.  We're just trying to get some moisture into the area in question.

Anyhow, the combination of high moisture followed by a good blast or two from the bulb blower will normally get anything that would be any problem.  It's fortunate that only huge dust or lint on the mirror will be visible, and such large pieces are the easiest to blow off.

Bulb Blower
Again, if all else fails you may feel that using a brush or other implement touching the mirror is necessary.  You do this at your own risk, of course.  I'm not saying it can't be done.  I've cleaned a "haze" of gunk (long story) off of mine with a cotton swab and solvent.  But I am saying that it's potentially risky, and you know what the consequences might be when you undertake such a procedure.  It is almost never necessary and must be done with extreme caution.  This is something you might be better off having done by an authorized repair facility.

Using a bulb blower to blow out the camera.

Focusing screen dust:

If the dust is visible through the viewfinder and is sharp and distinct, it is almost certain to be on the focusing screen.  Thus, you may not care to do anything about it.  After all, it won't affect your photos.

On the other hand, it might be irritating to look at and could possibly (not likely) cause metering errors since the Canon DSLRs meter by looking at the top of the focusing screen just the way we look at it through the viewfinder.  You would need to have an awful lot of dust or huge chunks for this to be any concern.

But let's say that this visible dust bothers you enough to want to get rid of it.  How do we proceed?

The dust is most likely to be on the underside of the focusing screen because that side is exposed to the interior of the mirror box.  You could have dust on the top of the focusing screen, but it's far less likely.

Focusing Screen

The focusing screen is shown in this view through the lens opening looking upward.  The screen forms the "ceiling" of the mirror box.

The focusing screen is made of plastic in most of these cameras and it is quite easy to scratch or mar.  So touching it with anything is very risky.  The good thing is that if you do scratch it, it's just the focusing screen and it won't directly affect your photos - just like the dust on it doesn't!

Still, I advise that you don't touch the screen with anything and instead use a bulb blower to blast the underside of it to try to dislodge any dust.  The bottom of the focusing screen is visible and accessible by simply removing the camera's lens.  You DO NOT want to use the camera's sensor clean mode for this because the mirror will flip up and cover the screen!

So just take off the lens and then blow the underside of the screen with your bulb blower and see if that doesn't get the dust off.  Sometimes static electricity will hold the dust to the plastic screen and in those cases, simply breathing (again, gently as described above) into the open camera may provide enough humidity to dissipate the static charge and allow the next blast from your bulb blower to get the job done.

You could use a soft brush to try to knock stubborn dust off of the focusing screen, but I've never found that necessary and you risk damaging things this way.  And rubbing most brush bristles across the plastic can impart an even greater static charge to the screen which then attracts and holds dust even better!

Remember that dust on the focusing screen won't show in your shots anyhow.  Why get too worried about it?  It's purely cosmetic.

Sensor Dust:

Finally, we get to the main point of this all!  Dust on the filter (number 8 in our drawing) ahead of the sensor can cast shadows onto the sensor when we take a picture.  This is really the only place that dust can normally be to cause visible dust marks on our photos.

The dust is held slightly away from the actual sensor by this filter so if we use a larger lens opening (small f/number), the dust may not be visible in the photos at all.  You can see how light from a larger area can find many paths to reach any given location on the sensor.  Many of these paths can "get around" the dust speck.  But when we stop the lens down to a small aperture (high f/number), the light must pass through that small aperture.  And as this aperture becomes smaller, it reduces the size of the "light source" and thus the shadows become ever more distinct.

Dust testing:

To test for dust on the sensor filter, take a photograph of a bland background (I like a white wall or piece of paper, but you can also use a clear-blue sky).  Take the photo with the lens adjusted so that the wall, paper, or sky is out of focus (we don't want to see dirt on the wall, marks on the paper, or birds in the sky).  And use the smallest aperture (highest f/number) for the lens.  This will make sensor dust show up easily.

Many people process their test shots in a photo editing program and turn up the contrast or do an "auto levels" adjustment to make the dust more distinct.  You can do that if you want to, but you may not like what you see :)

For me, I'm just interested in detecting dust that will be likely to affect "real" photos that I'll be taking.  So I just take the test shot and then view it on the rear LCD of the camera while zoomed in and "panning around" to cover the whole frame.  Anything that shows up when doing this is something that I want to get rid of because it is likely to show up in real photos.

Dust Test

Here's what some typical sensor dust of mine looks like zoomed in while reviewing one of my dust test shots:

My camera's "Multi Controller" gets a workout as I scan the zoomed area around to inspect the entire frame at high magnification.  The alternative is to unload the photo to your computer and view it on screen.  I find this method to be faster because you can switch between cleaning and testing in seconds.

Oh, Darn!  I've got dust!  What shall I do?

Once you've established that you have dust on your sensor filter that will affect your photos, you're faced with the question of what to do about it.

First, read the manual that came with your camera.  The manufacturer will detail their approved method for cleaning dust from the sensor filter.  Read these directions carefully and do what they recommend.

For the Canon 20D, the first recommendation is to install a freshly charged battery or use the AC power adapter.  I recommend using the battery because I don't like the idea of what would happen if I had a sudden power failure (mains failure) while cleaning my sensor filter.  I also don't like the idea of using the battery grip since there have been reports of intermittent power contact when using the grip.  So for me, I put a "known good" and freshly charged battery directly into the camera and then proceed.

Next, the 20D manual says to remove the lens from the body.  That makes sense.  I don't know how you'd do any cleaning with it in place!

Then you are to put the camera into the sensor clean mode.  When you do that, the mirror pops up and the shutter flies open and they both stay that way.

You can now look into the body and see the sensor through the stack of filters.  It's an amazing sight.  That's a huge integrated circuit.  No wonder the darn things are so expensive!  Check it out.  It's worth a look even if you don't need to clean it.

The Sensor Revealed

With the camera in the Sensor Clean mode, the mirror flips up and the shutter opens.  Thus, the sensor is visible through it's stack of filters.

On the 20D, if the battery is too discharged for safe sensor cleaning, the camera will not go into the clean mode.  If the battery gets low while the camera is in the cleaning mode, the camera will beep to warn you that the power may fail.

What's the big deal with running out of power while in the sensor clean mode?

The camera requires power to hold the shutter open and to hold the mirror up.  If you were to lose power while cleaning the camera, and if you had something inserted into the body of the camera, the shutter might try to slam closed on that tool and the mirror might also flop down onto it.  Thus, the shutter and/or the mirror could be damaged.

The manual for the 20D tells you NOT to insert the nose of your blower beyond the lens mount, so you'd be safe even if you did lose power.  But the fact is that some cleaning methods require you to insert a tool into the body so that you can wipe the sensor directly.  In that case, a power loss could be very dangerous.

Blower With the camera in the sensor clean mode, hold the body so that the lens mount is aimed down and blow upwards into the camera to dislodge as much dust as possible.  This is the Canon recommended cleaning technique.

This is the exact same method used to clean the mirror and focusing screen.  That's why this photo looks so familiar.  Blow the sensor vigorously and repeatedly with the blower.  You want to blow the dust off of the sensor and completely out of the camera.  Give it a bunch of fast, hard pumps and then switch the camera off.  That will take it out of the sensor clean mode and you can then install the lens again.  I often blow off the back of the lens right about now and then put it onto the camera quickly to prevent more dust from getting into the body.

A bulb type blower aimed upwards into the camera.

Now, you should perform another dust test shot.  Check that shot and if all is well, then you're done!  According to Canon, this is all that you will ever need to do to get your sensor clean.  Often this is true.

Oh oh.  There's still dust on my sensor filter!

OK, don't panic.  A lot of times, just blowing the camera does not get all of the dust off of the sensor filter.  This is because the dust is stuck there.  Why is it stuck there?  Good question!

If you exposed the camera to something wet, like changing lenses while at the beach with some nice salt spray on the breeze, then there may be residue from that on the sensor filter.  This isn't very likely, though, because even when you're changing lenses, the shutter is closed so anything that gets into the body won't get directly onto the sensor filter.  Instead, that salt spray should just get on other things in the camera and dry there.  So at the worst, you've contaminated the body with salt residue which can break free later and act as any other dust.  Not too bad.

But there do seem to be things that will stick to the sensor filter and for whatever reason, they need to be washed off.

But we'll start with the most likely reason that your dust didn't just blow off of the filter.  Think about what this filter is made of.  It's glass of some sort.  And it most likely has some anti reflective coatings applied to it.  Since Canon and the other manufacturers don't tell us what these coatings are, we are left to speculate.  But it's possible that they are materials that tend to be prone to holding a static electric charge.  Bare glass is pretty good at that all by itself and the coatings may be worse.

So it's reasonable to think that the sensor filter may have a static charge on it.  And if you combine that with dust made of materials that also tend to accumulate a surface charge, you've got a recipe for "stuck dust".  Pollen is often regarded as being a problem because it's tiny and very static-prone.  All the blowing in the world may not knock things like that free from a highly charged filter.

So one thing you can try that is very safe and easy is to simply breathe into the camera while it's in the sensor clean mode to raise the humidity in there.  Once again, let me repeat that I don't suggest "blowing" into the camera.  Blowing implies making the air come out of your lips at a high velocity.  High velocity air can easily pick up droplets of spittle and then they'll fly in and land on your sensor filter.  That's not what we want.

Instead, breathe out gently and slowly with your mouth open fairly wide so that your warm, high-humidity breath just slowly enters the camera.  The moisture from your breath is what we want.  Not any kind of blowing action.

The humidity from your breath will often allow the static charges on the dust and the sensor filter to equalize and dissipate.  Then, a good standard blowing with the hand blower may well get the job done.  Give this a try.

Again, don't spit, wheeze, cough, or blow into the camera and onto the sensor.  Just breathe slowly into the camera to get some moist air in there.

Now you need to test again for dust.  You might have success and it's pretty easy to try it.  If that got things cleaned out to your satisfaction, you're done!

NEW:  The Canon 1DMkIII, 40D, and 400D are reputed to now have a Tin Oxide coating (actually, I suspect that it's more likely to be a tin-doped indium oxide coating) on their sensor filters to eliminate static.  This sounds like a good idea to me and should help with dust.  Thus, it may not do any good to breathe into these bodies to try to dissipate the static charge on the filter.  But it might still help to discharge the dust itself.  It's hard to know.  I don't think breathing into those cameras will do any harm, however, so it's probably still worth a try.  The 5D also has a different front surface on its sensor filter.  But it's not the same as that on these newer cameras.  We don't know much about that one.

Oh darn!  There's STILL dust on my sensor filter!

Alright.  It's OK.  You're now where most of us have been at times.  You've done what the manual recommends and it didn't get it all.  Next, you've tried a bit of humidity to help out, and maybe that didn't get the job done either.

Now you've got to ask yourself a question or two:

Do I dare do something that could damage my camera's sensor filter in order to get the dust off?  Or do I need to send/take the camera to Canon to have it done?

That is a tough question because you are only authorized by Canon do use the blower.  If that didn't get the job done, and you don't want to potentially affect your warranty coverage, you are done.  You'll just have to live with the dust you have or send/take the camera to Canon to have them clean it.

I can't tell you what decision to make at this point.  I know what I do and I also know that if I wasn't willing to go further on my own, the camera would be nearly useless to me.  I get dust on my sensor filter frequently.  Where I live it's dry and dusty.  I have six cats and I clean the box.  That stuff is dusty!  And to top it all off, I shoot a lot of macro photos and that requires the use of very small apertures at times.  So I see any little bit of dust in many of my photos.

As a result of all of those factors, I have arrived at a fairly fast, easy, and cheap way to get my sensor filter clean.  I do it frequently.  Probably an average of once or twice a week, but timing means nothing.  It all depends on what I've exposed the camera to and what I'll be shooting.  If I'm about to shoot macro, I test and clean as necessary because I'd rather spend five minutes cleaning my camera now than spend five minutes per photo "cloning out" the dust spots later.  It's just simple time management.

And I live in the middle of Wyoming.  We don't have an authorized Canon repair station nearby.  The nearest one is in California, I believe.  Imagine me sending my camera in for cleaning twice a week.  Does that sound practical?  How many bodies would I need to own in order to keep a constant flow of bodies in transit between me and Canon?

So I clean it myself.  I take full responsibility for damaging my sensor filter.  I know it'll cost a bunch to have it replaced if it is damaged.  I accept that and proceed with care and attention to detail, but in a fast, cheap, expedient way.

The rest of this article describes what I do.  I can't authorize you to do it this way.  In fact, I'll tell you again right now that this is not authorized by Canon and any damage you do to your camera is your responsibility.

You've been warned!

Read the additional warnings at the end of this page too!

If you have one of the newer cameras with anti-dust features or if you have a 5D, REALLY read those warnings carefully!


Many fine tools are available to clean your DSLR's sensor filter.  Most of these involve the use of a swab of some sort which has a flat profile.  The swab is moistened with a small amount of Methanol and is then drawn across the sensor carefully in one pass to wipe the dust off and collect it.  These systems work very well from all of the reports I've read.

Another common method is to use a brush of some sort to brush the dust away.  The brushes are often pre-charged with static electricity to make them more attractive to the dust than the sensor filter so that the brush will collect the dust rather than simply push it around.  Again, from reports I've read, these brush methods work fine too.

But I wanted something easier to use and less expensive so I did the unthinkable and tried plain old generic cotton swabs.  How horrifying!

Those can't work.  They aren't high tech.  They don't cost enough.  It's just too darn easy!

Well, I'm sure these other methods have advantages.  And they're well tested and loved by many.  So I'm sure they're just great.

But the actual thing touching the sensor filter with any of these other methods is, in every case, something at least as "hard" as cotton.

In the case of the various swabs, the material is usually cellulose.  They are a highly refined paper.  Cotton is another form of cellulose, and is often regarded as being softer than paper products.  Hmmm, that seems OK.

So where is the danger with any of these methods?


That's what you need to beware of.  If you use any of these "touching the sensor" methods, and you happen to have a particle of hard grit (sand or dirt) on the sensor, you run the risk of forcing that grit against the filter and dragging it across it.  That would surely scratch the filter.  It wouldn't matter what your swab was made of, or even if you were using a brush.  If you drag a particle of hard, sharp grit across the sensor filter, you may scratch it.

So regardless of what you use as a swab, your enemy is grit.  This is why it's very good practice to pre-clean your sensor carefully with the blower before using any "touching the sensor" method.  Always try the blower first.  Only move on to any of these swabbing or brushing methods if you've already blown the sensor filter off well and tested to be sure the swabbing/brushing is required.

I buy generic cotton swabs at the local drug store and I keep them in sealed zip-top plastic bags containing about 20 swabs each.  That way, I can keep them clean and if I think a bag of them may have gotten contaminated, I just throw the whole bag away.  These things are cheap.  There is no sense risking the use of a contaminated swab when they cost a half a cent apiece!

And on the subject of contamination of the swabs:  When you grab a swab out of the bag, decide which end you'll be using to do the swabbing, and which end will be in your hand.  You don't want to touch the end that you'll be using for the swabbing because your skin's oils and dead cells, etc., will contaminate the swab and end up smearing your sensor filter.

Keep it clean.

One more thing about the swabs:

I have found that these generic swabs from the local drug store work better than the "brand name" swabs or the special "high-quality" swabs often sold for delicate cleaning operations.  The reason is that the generic swabs' heads are wound looser.  The fancy swabs often have their heads wound very tight.  This may reduce fiber shedding, but it also means that the tip of the swab is small and tight.  The looser-wound generic swabs cover more area in a pass than the tight, hard ones do.  This makes it easier to cover the area quickly in less passes and it allows the end of the swab to mash down with very little force applied.  The loose swabs get into corners better too.

So while it might be tempting to use the very high quality individually packaged swabs because they should be cleaner, etc., it has been my experience that they don't work as well because they are not as compliant.  So the best swabs I've found are the generic ones from the drug store.  I suspect that there can be a lot of variation from brand to brand.  Try a few different brands and find one that works for you.  You want a swab tip that is soft and fluffy not hard and tight.  You'll be mopping the sensor filter.  Imagine mopping a floor with the end of a stick - wouldn't work very well, would it?  You want the mop head to cover some surface area without needing to apply a lot of force :)


It is possible to wrap a Pec Pad around a cotton swab to make a handy swabbing tool too.  So if you'd rather have a Pec Pad in contact with your sensor filter, that's an option.  But I've had good luck with the raw swabs so take that all for what it's worth.

OK, OK.  We're ready to swab, already.

First, I assume that your camera is at normal room temperature of around 20 degrees C (68 degrees F).

It's nice to have a good, bright lamp to work under.  I like those swing-arm lamps because you can position them for good viewing.  But this isn't completely necessary.

Take the lens off of your camera.  Set it aside.

Take out a swab or two and set them down on a clean surface remembering which ends are the "clean" ends.

Now, I'm right-handed so I'll describe this as if you were too.  If you're a leftie, just use the other hands.  No problem.

Put the camera into the sensor clean mode and turn it up so that you can see the sensor.  Of course, you're actually looking through the clear filter, and that filter is what you'll be cleaning, but you get the idea.

Hold the camera in your left hand so that the opening is facing up and you can see into it nice and easy.

If you get the camera and the light just right, you can see the whole surface of the filter, and you can rock the camera around to get the glare from the light to reflect off of that surface.  That makes it easy to look for dust, smudges, and other stuff.

You won't be able to see the dust, usually.  If it's a whopper, or a piece of lint, you might, but generally, by the time you've blown the camera out a few times, you've gotten rid of anything large enough for you to see.  But check it out anyhow!  If you see anything that looks like it could be a piece of hard grit on there, you need to think about how to get it knocked and blown off of there before you do the swabbing.

You can poke at it gently with a clean swab and see if it'll move and then blow it off.  (Pitch the swab and get a clean one now).

OK, everything looks good?  You're ready to swab.

Pick up one of the swabs in your right hand being careful not to touch the "business end" of it.

Hold the camera up and gently breathe (once again, do not blow, sneeze, cough, or wheeze - just breathe out slowly with your mouth fairly wide open) into the body.  You will see a fog of condensed moisture form on the sensor filter.

Breath Fog
That fog is actually condensed water.  By definition, it is distilled water.  And further, it's been fractionally distilled because the temperature of the inside of your lungs is only a few tens of degrees warmer than the surface onto which the water condensed.

Thus, this is extremely pure water.  A friend of mine who is a chemist pointed out to me that there will also be other gas components of your breath which will end up dissolved into that water too.  One main component of note is carbon dioxide.  That CO2 will form carbonic acid in this water.

Those acid ions will make the distilled water (a good insulator) into a reasonable conductor.  Thus, this slightly acidified water will work well to equalize and dissipate the surface charge of the filter with respect to any dust that is on it.  This will make it easier for the swab to remove the dust from the filter.

Canon 20D camera sensor with its filter "fogged" over with condensed breath.

With the filter fogged as shown, you're ready to swab.  You've got just the right amount of water to do the job.  It won't be so much that you are in any danger of having it run in anywhere yet there is enough there to let you "mop up" the dust.

This photo doesn't show the ideal pattern for swabbing the sensor because I had to take these shots with an uncooperative P&S camera in one hand and the swab in the other while holding the shutter button at half-press after trying to lock the focus.....  Well, you get the idea.

So this photo just shows how the condensation is picked up by the swab as you rub it gently over the surface of the filter.

In actual practice, I use a "raster" pattern to scan over the whole surface and then I run the swab around the edges a few "laps" to get the dust that likes to hide in the corners and along the edges.

I don't use much pressure, and I don't need to because unlike with the paper covered spatula technique, I can get the whole surface in due time and I can see where I've been because of the moisture that is being smeared around as I do this.

By looking at the filter as you swab it, you can see where you've been and you can see when the water has been polished off of the surface.  When the water is gone, you're done.

Swab removing the water as it cleans the surface.

Dry Filter
This shot shows the swab against a dry filter.  When it's this dry, you don't need or want to continue with the swabbing.

Yeah, the swab has mysteriously moved to my left hand.  I'm sorry, just a continuity error there.  Again, I'm shooting the other camera one-handed so that's the way it goes.  Let's say this one is for the left-handed folks out there.

The filter is dry, so you're done swabbing.

Swab pattern
I run the swab back and forth over the sensor in a pattern like this to get the whole surface clean.

I can see where I've been by the patterns left in the water on the surface.

The final step for me is to run the swab around the outside perimeter of the sensor, up against the edges of the "well" that it sits in.  I often make a few laps around the perimeter like this since that's where a lot of the dust seems to end up.

One possible swab wiping pattern.

The main thing is to get the whole surface wiped while there is some moisture there to help out.  As the moisture is spread around and evaporates, you can see where you've been and see if the surface looks clean.  Once the water is gone, and the surface is shiny and free of streaks, you're done.

The whole process takes about ten seconds.

Throw the swab away.  If you decide to swab again, you should use a clean swab.  Hey, they're ridiculously inexpensive!

The final blow:

Now that you've swabbed the whole sensor, you have one final thing to do.  Get the bulb blower and again hold the camera so that the lens mount is aimed down.  Do another good, vigorous blowing to get any loose fibers that may have shed from the swab and also any crud that may have fallen into the camera while it was aimed upwards.  Blast it good!

Now, turn the camera off to take it out of sensor clean mode, and put your lens back on.

You may now test for dust once again.

It may not be perfect at this point, but I'll bet it's pretty darn close.  And if it's not, you know just how easy it is to simply do the same thing again.  Sometimes it takes a couple of swabbings.  That's fine.  It's cheap and fast.

As you do this a few times, you'll get very good at it and become more comfortable with the whole process.  You don't need to press hard at all because the swab conforms to the corners and the surface.  And you can get into all of the edges and corners with ease.

There is one protrusion on the left side of the "sensor well" that is kind of in the way as you swab (on a 20D), but you soon get a feel for where it is and it's no trouble to angle the swab to avoid it so that it doesn't interrupt the smooth motion of the swab.

A nice, gentle, smooth wipe seems to work the best.  Bring the swab into contact with the filter just once and keep it in contact for the whole wipe if possible.  That seems to get the dust the best.

You'll get a feel for it after a few tries.  Don't expect perfection the very first time.  But also don't expect this to be difficult or hard to learn.

Other solvents:

(Read the warning below concerning new cameras with special anti-dust coatings).

I have gotten oil on my 20D sensor filter before when using a brush to clean it.  I must have touched something in the camera that had some lubricant on it and then transferred that lube to the sensor.  That sounds nasty, but really it wasn't much of a problem at all.  I simply used some alcohol and a swab to remove the oil.  Various alcohols are available for this use.  The most common one used in the commercial sensor cleaning preparations is methanol.  That works fine on the 20D.  See below for other bodies.

However, another good, clean source for a cleaning alcohol is to purchase a new bottle of "Everclear" at the liquor store.  Everclear is 95% pure grain alcohol (ethanol).  It seems to be very pure and free of impurities that would leave a reside.  I've used it a number of times when I wanted a less polar solvent than water.  For greases and oils, it is reasonably effective.

Again, use it at your risk, and only use clean Everclear from a fresh bottle.  You may wish to keep a bottle just for that purpose so that it does not become contaminated.

Use only a very small amount on the tip of the swab.  You DO NOT want the swab to have so much that it could drip or run anywhere!  That could be a very bad thing indeed!  Use the same method as you'd use with the condensed breath.  You can see the alcohol left behind by the swab as you move it over the surface.  For nasty oil or other residue, you may need to do this several times.  Use a fresh swab for each attempt.  It's all very cheap.

The bit of residue left behind after the Everclear swabbing can be removed with a followup swabbing with the condensed breath as the final polish.

It has been said on another website about DSLR sensor cleaning that Canon uses a mixture of alcohol and glass cleaner to clean these sensors.  I cannot verify that.  But this brings up the idea of using various things to clean the sensor filter.  My warning about that would be that if you use the wrong solvent, you may damage or remove the special coatings that are on the sensor filter.

Pure water and methanol have been shown to be safe on the 20D, and I have seen no problems using the ethanol from Everclear.  That's all I can say about that.

An additional warning!
Anti-Static sensor filters and such....

NEW:  As mentioned above, some newer cameras with anti-dust features are reputed to have a tin oxide (or more likely tin-doped indium oxide) coating on the front surface of the sensor filter.  The manufacturers of some sensor cleaning solvents have come out with newer solvents made especially for these cameras.  I do not know what the new solvents are exactly, but reports I've read say that the MSDS for one such new "Tin Oxide Compatible" solvent is mostly ethanol denatured with some methanol to make it undrinkable.

The fact that they've come out with a new product suggests that they have found some problem with using their other solvent (pure methanol) on these newer cameras.  They were recommending the new non-methanol solvent for the Canon 400D, 5D, 1DMkIII, and 40D.  Now I see that they've gone back to recommending the methanol solvent on the 5D.  I've read of problems with cleaning the 5D.  Reports have been seen of the coating on the sensor filter of the 5D flaking off!  Speculation was that the 5D simply used it's "hot filter" coating as the front surface.  That coating may be extremely fragile.

If I had a 5D, I'd be VERY careful using anything to clean its sensor.  They may well be far more delicate than any of the other Canons.  We just don't know for sure.

It appears that
the new non-methanol solvents are indicated for NEW model Canon cameras with the "anti-dust" features.  So be careful when choosing a solvent to try on one of these newer cameras.  There must be some reason why people are avoiding methanol.

As for the scratch resistance of these coatings:  I do not know.  It may be more delicate than what we formerly faced on these filters.  I recently got a 40D but have NOT done extensive testing.  I've cleaned it twice with the condensed breath and swab method, and it seems to have worked just fine with no ill effects.  But I DO NOT have the extensive experience with it that I do have with the 20D.  So take that for what it's worth!

Since the manufacturers of several cleaning systems are not telling people to avoid using their methods on these new bodies, I have to believe that they feel that the new sensor filter coating is not overly delicate.  Also, a bit of reading about tin-doped indium oxide coatings indicates that these coatings are sometimes used to increase the wear resistance of glass.  Thus, one would hope that the coating is harder than glass which would make it less likely to scratch.

But we do not know exactly what this coating actually is, how it is applied, or whether it might be beneath some other coatings which might still be very delicate or which, perhaps, do not stick to it as well as they do to glass.  So the whole assembly might be more delecate or more susceptible to damage from solvents and rubbing.  I simply do not know!

You really are doing any of this entirely at your own risk, including using the commercially available swabs and solvents!!

One last disclaimer:

This method is not authorized by Canon.  According to Canon, you are only allowed to blow your sensor off.  Be sure to try that first every time.  And if you decide to attempt my method, don't blame me if you damage your camera.  I take no responsibility for anything you might do with this information.  It works for me, and I've done it hundreds of times on the 20D with no problem.  But I have no idea how competent anyone else is and I cannot test it on other cameras so I can't assume any responsibility for your results.  This is probably why Canon only authorizes us to use a blower.  They cannot know if we'll end up damaging our cameras with some other method so they just leave us with only the blower.

I have read a couple of horror stories of people scratching their camera's sensor filter using the "normal" sensor wiping methods.  These reports are infrequent but not unheard of.  And I've read about bad experiences with the 5D's coating being totally removed!  Any sensor cleaning that involves touching the sensor with anything carries some risk.

Happy shooting everyone!  A clean sensor is a happy sensor :)