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Sensor Dust Cleaning
Unlike film cameras, current digital SLRs suffer from a frustrating weakness: sensor dust. Just how bad is this issue? It's enough of a problem that I bought an extra digital SLR for my Africa trip! The following page summarizes the issue, and describes a method I've used to correct the problem.
Sensor Dust - What is it?
CMOS sensor exposed to view
The very nature of an SLR camera - the removable lens - opens up the flexibility of multiple lens choices, but it also opens up the inner chamber to the outside environment. When these small dust particles (smaller than you can see with your eye) settle inside the camera in that short period of time that you use to change your lens, there is a good chance that they'll immediately jump onto the CMOS or CCD sensor as soon as you turn the power on (charged surface will attract the particles).
The term "sensor" on this page is used loosely, as the dust isn't actually sitting on the sensor itself, but an element that is just in front of it. These elements include the dichroic mirror or low-pass filter.
These specs of dust, hair, lint and everything else cling to a surface in front of the sensor, and will be seen in many of the images that you take. As these particles are not on the sensor itself, but a little distance in front of it, a small aperture is required to bring them into focus -- a large aperture will allow light to "wrap around" the particles, essentially removing them from view.
Sometimes you see it, sometimes you don't
The effects of "sensor dust" are observed in photos taken with small apertures (ie. large depth of field, eg. f16, f22, etc.) The effects become less noticeable with larger apertures.
As many people shoot wide open (ie. with as large an aperture as possible, to increase available light and therefore shutter speed), the nuisance dust is often invisible. But, the day that you decide to take that one landscape photo with a nice blue sky (with large depth of field), you suddenly see all these dark specs across the sky!
Why is it a big deal?
Perhaps the most frustrating thing about sensor dust is that it is cumulative! Every time that you open up your camera body (ie. change your lenses), there is a good chance that you've just added a little more dust to your sensor. Dust never disappears, so over time the problem gets worse and worse!
Film versus Digital - Sensor Dust
You might be asking why it is that this dust issue isn't a problem for film cameras... They are just as susceptable to dust, but the dust settles on the negative itself. The frame that caught the dust will be affected by the visible effects much in the same way as on a digital SLR (often worse because it will be clearly in focus). The difference is that when you wind on the next frame, you get a fresh negative frame free from dust. Typically, it's only the first images after you expose the mirror chamber to dust that will show the symptoms.
Future solutions for Digital SLRs?
As more and more consumers become aware of the problem, there will be a natural push for dSLR manufacturers to develop unique solutions to the problem. To date, there are very few digital SLRs that have shown innovative solutions. A few that come to mind are:
- Sigma - Image Sensor Dust Protector
A removable clear plate that sits above the sensor, allowing a first surface for dust to settle upon, reducing the visible defects. The dust on the protector is rarely visible because it is far from the image plane, and it effectively seals the mirror box and also protects the reflex mirror and viewing screen (according to Sigma). Incorporated into the SD9 and SD10 cameras.
- Olympus - Ultrasonic Sensor Vibration
Called the Supersonic Wave Filter (or Dust Reduction System), Olympus introduces a unique approach that involves adding a filter in front of the CCD, one that uses ultrasonic vibration to free itself of dust just prior to shutter release. The Olympus E-1 was the first camera to feature this unique approach.
- Nikon - Software Remapping
The Nikon Capture software that is sold by Nikon introduces a feature called Image Dust Off. The software attempts to map out a sensor dust pattern with help from the user and a reference image. It then applies a correction to all images in a batch process. Of course this solution is a weak one, and one that any manufacturer or software company can provide. The problem should be addressed at the source (ie. the mirror chamber) instead of afterwards in software. The Nikon software to utilize the Dust Reference Image data costs $100.
- Sony - Vibration
Introduced with Sony's first dSLR, the Alpha / DSLR-A100K, a combination of anti-dust coatings is used along with a CCD vibration mechanism.
- Canon - EOS Integrated Cleaning System
On the new Canon Digital Rebel XTi (or Canon 400D), Canon has introduced their own version of a dust-reduction solution that involves anti-static coatings, vibration-cleaning (of the low-pass filter) and software mapping-out of stubborn particles. Yet approach Canon took with the Rebel XTi was moving the front of the low-pass filter (used for anti-aliasing / reduction of moire) out by another millimeter or two. The net effect of shifting the filter element forward is that it is more likely to throw dust particles further out of focus. The mapping process is dubbed the Dust Delete Data system and is easily configured by shooting a photo of a blank white piece of paper. After interpretation by the camera, the dust map is stored in the camera's non-volatile memory where it will be written into the metadata of all subsequent photos. Software on the PC (Canon's Digital Photo Professional 2.2, free) then uses this information to "blend in the density" of the affected areas (essentially cloning the particles out).
Canon's Sensor Dust Cleaning in Digital Rebel XTi
How effective are these camera solutions?
Unfortunately, it seems that some users have found that these built-in methods are not as effective as all the glossy marketing may imply. Have a look at Testing Dust Removal Systems fore more.
This page is intended as a rough guide to introduce the issues and potential options. I assume no liability for any damage (operational or warranty) caused by the use of any methods listed on this page. The reader is fully responsible for doing adequate research on the various solutions before undertaking any of them!
Assuming your camera has not provided you with a great solution to the dust issue, you are left facing the predicament of how to fix it. The option that most people take are:
- Ignoring the problem
Many people are not even aware of the problem because they rarely shoot with large depth of fields. Or, they notice the problem and choose to overlook the little specs. Perhaps some people can do this, but there's no way that these little distractions sit there.
- Correction in software
Perhaps the most time-consuming of all approaches is the one most people naturally end up doing if they take the rare small-aperture photo. In Photoshop CS, it is a relatively easy task to use the healing brush (or clone stamp tool in prior versions) to stamp out the dust specs, one at a time. If you are taking many photos that suffer from this problem, you're probably going to go crazy. Some software utilities (like Nikon's Capture) allow you to batch automate the process to a degree.
- Sending to manufacturer
All manufacturers are going to encourage this solution as it guarantees that the operation will be done carefully, won't void your warranty and will increase their revenues. Unfortunately, this can be reasonably costly, inconvenient and can set you apart from your camera for a period of time. In environments where you are going to experience dust issues regularly, or places where you are far from a service centre, this option becomes less appealing.
- Air-based cleaning
Many people try to use air-based methods to clean their sensor. Although you run far less risk with scratching the sensor as the next two options, there can be some potential damage from using compressed air / CO2. Also, you will find that many of the dust specs are truly stuck to the sensor and blowing air across them won't do anything to remove the stubborn pieces.
- Sensor swabbing (wet method)
More and more, sensor swabbing is becoming a very popular do-it-yourself approach to sensor cleaning. There are some extremely good articles written on how to do this safely, and I cannot do them justice by paraphrasing the techniques here. One of the most thorough resources on the topic is provided by Nicholas R in the Copperhill CCD / CMOS Sensor Cleaning pages, which is based on the use of Eclipse solution (methanol), a SensorSwipe and lint-free Pec*Pads.
- Sensor brushing (dry method)
Another very popular method is to use a special brush with very fine fibers to clean the surface. The fibers are charged to help carry the dust away from the surface, without having to use a cleaning solution. The most notable product in this category is the Sensor Brush by Visible Dust. An obvious advantage versus the wet methods is that no cleaning fluids need to be used.
- Unusual approaches
It appears that there are a number of people posting in user forums who are using Scotch Magic Tape to clean their sensor. Apparently people are seeing success with this approach, even though I would expect it to leave a residue behind. I cannot recommend this approach, but have yet to see any postings regarding residue or damage occuring from this method. My hunch is that very few people are using it. Read a forum post about the Scotch Magic Tape method.
- Terrible approaches
All that being said, there are some very creative and terribly destructive approaches that have been floating around the internet. They range from everything including using a vacuum cleaner inside the mirror chamber. I hope my readers are intelligent enough to see the humor in these disastrous but creative solutions.
What did I do?
With a trip to Africa coming up within a week, it was time for me to bite the bullet and use the cleaning kit that I had ordered from Copperhill ages ago. At the time it was a toss-up between the dry brush technique and the wet swabbing technique -- in the end I chose the Copperhill kit, but I am sure that the Sensor Brush method would have worked equally well. To be honest, I was quite nervous about doing a sensor cleaning, and kept putting it off until I realized it was time to do it. I had collected a significant amount of dust in my Canon 10d and a moderate amount in my Canon Digital Rebel / 300d.
Realizing that Africa would be extremely dusty, I was faced with a serious problem: how to use multiple lenses in a dusty environment where one cannot hope to do a proper sensor cleaning? Similarly, I discovered that I couldn't ship the cleaning solution (methanol) on the trip, so I would only be able to clean while still at home.
The solution? I bought three digital SLRs instead of two (one is the standard backup, or should I say my wife's!). My plan is to have my two main lenses continuously mounted to the two cameras, that way I never have to change lenses in the [dusty] field. My Canon 100-400/L IS is mounted on my Canon 10d and the Canon 17-40/4L is on the Digital Rebel. By never changing lenses, the introduction of dust into each camera is reduced significantly. Unfortunately, the Canon 100-400 is a telescoping lens (ie. no internal zoom) and so it is expected that it will pull some dust through the chamber by the very nature of its changing volume. I will have to see what really happens in the field.
Then, it was time to do the cleaning...
Copperhill's Sensor Cleaning Kit
Following the directions from the Copperhill website carefully, I tested out the approach on the Digital Rebel first (I was going to experiment on my wife's Digital Rebel first, but I didn't feel right doing that!) It took far less time than I expected (half an hour), and you can see the tremendous improvements shown below.
The images below show the degree of dust on my sensor before cleaning and after. These images are highly modified to reveal the dust patterns. In photos, these particles are only visible in very small apertures (eg. f22). The images were created by photographing a [roughly] even illuminated white card at f22 at ISO 100. In Photoshop CS, I then used Auto-Levels which will introduce an extreme degree of contrast into the image, making the dust specs stand out. Then, I desaturated to remove the irrelevant color shifts.
As one can see in the images above, I had a large number of dust specs on the sensor prior to cleaning. After cleaning, I was left with only a couple, typically around the outer edge of the sensor.
I repeated the cleaning process twice for both of my cameras, and if I had done it again, it is very likely that I would have removed nearly all of the annoying specs.
Steps for Dust Prevention
- Don't change lenses in dusty environments.
- Always turn off your camera before changing lenses. The CCD or CMOS sensor will be charged when on, and it will be attract dust much more easily in this mode.
- When you change your lens, don't remove the dust cap from the new lens until after you have removed the old lens. Immediately use the dust cap on the old lens (or stand it mount-end down) and then swap.
- Try to keep the camera chamber pointing down while changing. Similarly, try to keep the camera-side of the lens pointing downwards when handling it. If it is windy, turn away from the wind and use your body as a natural shield.
- Consider a longer-range zoom instead of smaller-coverage zooms if you will be working in dusty conditions for extended periods of time.
- Buy an additional camera so that you can leave your short lens mounted on one camera and the long lens mounted on the either (my current solution for Africa).
Other notes and experiences
The following are other articles I have come across that might throw some more objective light on these techniques.