Have you ever noticed small darkish patches in areas of blue sky in landscape photos taken with your digital SLR? They’re probably produced by dust on the image sensor. In most cases, it’s not on the sensor itself, but on the filter or protective glass that covers it. The result is the same; it shows up on digital photos. Not all of them, though; in most cases, dust is only visible in shots with large areas that have minimal detail when the shot was taken with a small lens aperture.

 

Have you ever noticed small darkish patches in areas of blue sky in landscape photos taken with your digital SLR? They’re probably produced by dust on the image sensor. In most cases, it’s not on the sensor itself, but on the filter or protective glass that covers it. The result is the same; it shows up on digital photos. Not all of them, though; in most cases, dust is only visible in shots with large areas that have minimal detail when the shot was taken with a small lens aperture.

This kind of dust is only a problem with interchangeable-lens cameras because the inside of the camera body is not sealed off as it is in a camera with a built-in lens. It’s never found with fixed-lens cameras.

How dust gets in

Whenever you change a lens there’s a chance of dust entering the camera and, once inside the camera body, it is stirred up and re-distributed each time the mirror flips up and down when you take a shot. A build-up of static electricity is thought to cause the dust to be attracted to the sensor. Some cameras are more susceptible than others. Dust attraction has nothing to do with the type of sensor; CCDs and CMOS sensors can be equally dust-prone – or dust-resistant.

Essentially there are two kinds of dust to be concerned about: dust that simply sits on the sensor’s surface and dust that gets ‘welded’ on. The former is easily dislodged by systems that vibrate the sensor and can also be removed with a soft brush. Welded dust commonly occurs when a camera with dust on its sensor is moved between low and high humidity. Some of the particles absorb moisture, causing them to stick on the surface of the filter. Pollens are a common source of this type of dust, which requires more thorough sensor cleaning.

Most dust spots are very small; too small to see with the naked eye. However, even a tiny speck of dust will look large to a photosite with a surface area of approximately eight microns – which is typical for a 6-megapixel DSLR.

Fortunately, dust spots seldom show up unless:

  • The dust spot is located on a part of the sensor that records a bright area like the sky.
  • The shot is taken with a small lens aperture. At apertures up to f8 light strikes the sensor at varying angles and the shallow depth of field causes the dust speck to be unresolved. At f22, the speck will almost certainly be clear and sharp!

Detecting dust

Don’t expect to find dust by inspecting your DSLR’s sensor. A dust particle that can cover one or two photosites is essentially invisible to the naked eye and it’s a waste of time to look for them in this way. However, it’s quite easy to find out whether your sensor has a dust deposit.

1. Open Photoshop or Photoshop Elements and create a new image. Fill it with white and enlarge it until it fills your monitor screen.

2. Set the camera to Aperture Priority mode and the lens to the smallest aperture (f22 or smaller). Manually focus the camera on the screen, making sure the entire viewfinder area is filled with white. (Put up a cross or line on the screen to aid focusing and then delete it before taking the test shot.) Turn off any adjustments like sharpening, saturation, contrast, etc.

3. Photograph the white screen, adjusting the exposure to give a very pale grey image. Jiggling the camera slightly during the exposure ensures you aren’t taking a picture of marks on your monitor. Make sure the lens stays within the white target.

4. Open the test image in Photoshop or Photoshop Elements and apply Levels adjustment (either auto or manual). If you see dark spots it indicates dust.

Care is required when interpreting the result of this simple test, especially if you use Auto Levels instead of adjustign the Levels manually. Because of the way Auto Levels works, any imperfection will show up when you carry out this test, so unless you have an absolutely perfect surface to shoot, Auto Levels will find something to exaggerate – and you may see colour fringing in the background of the shot. If there are marks on the target surface, it’ll pick them up too! For that reason, the target MUST be absolutely blemish-free, flat and white.

Dealing with Dust

When your sensor has picked up only a few dust spots, it’s easy to remove them with the cloning or ‘healing brush’ tools in your editing software. You can then save the ‘repaired’ file and use it whenever you want to make a print.

However, once the dust build-up becomes significant and the time taken to ‘clean up’ shots becomes irksome, a more drastic solution is required. The best and safest solution is to take your camera to the distributor’s service centre and ask for the sensor to be cleaned. This is an expensive option but you have the assurance that the job will be done properly and your warranty will not be voided.

The next best solution is to try cleaning the sensor yourself. This should only be considered by those who are experienced in repairing and maintaining precision electrical or electronic equipment because most sensors are difficult to get at and, if you use incorrect tools you risk damaging the surface of the filter or cover glass. There’s also a risk of scratching the camera’s mirror.

You can find a wealth of information about sensor cleaning on the Internet; simply type ‘dust on image sensors’ into your browser. (Note: The products mentioned in some of the sites may not be available in Australia.) One of the best sites is http://cleaningdigitalcameras.com/, which has been produced by professional technicians. Canon Australia also provides instructions for cleaning sensors on its website in the FAQ section of its Customer Service pages.

It’s usually best to clean the whole sensor surface at once, but you may need to locate persistent spots for special attention, which means adjusting orientation. For example, a spot that is visible on the upper left corner of your test shot will be caused by dust in the lower right corner of the CCD as you face it.

Nikon distributor, Maxwell Optical Industries, offers one free CCD clean under warranty and will charge between $50-$70 per clean thereafter. Cost varies depending on the time it takes. If a customer is an NPS member (ie, they own two or more Pro Digital bodies and pay a membership fee of $85) they are entitled to two free CCD cleans per year.

Dust Prevention

There are a number of strategies you can use to prevent dust from accumulating.

1. Turn off the camera before you change lenses. This removes the electrical charge from the sensor, making it less likely to attract dust.

2. Make the change as quickly as you can; the shorter the time the inside of the camera body is open to the environment, the less time there is for dust to get in.

3. Avoid changing lenses in dusty conditions.

4. Don’t change lenses shortly after moving from an air-conditioned environment to humid outdoor conditions. Condensation is the main cause of welded dust spots.

5. ‘Pump’ your zoom lenses two or three times before mounting them on the camera to dislodge any dust that may have collected in the zoom mechanism.

6. Put the camera and lens you plan to fit into a clean, brand-new plastic bag. In theory, this should provide a dust-free environment for changing lenses. (In practice, however, it may amplify the build-up of static electricity so you should minimise the amount of handling the camera and lens receive when you use this strategy.)

7. Avoid shooting in burst mode.

8. Make sure you use body and lens caps when storing cameras and lenses and carry and store your camera and lenses in a good quality bag

9. Buy a camera with some kind of dust minimisation technology.

Dust Minimisation Systems

Olympus was the first camera manufacturer to install dust-reduction technology in its DSLR cameras, with a special Supersonic Wave Filter (SSWF) system that vibrates the filter in front of the sensor to dislodge dust. The dust collects in a tray below the sensor, which is cleaned when the camera is serviced.

Sony’s new DSLR-A100 camera has a dual-action dust reduction system that uses the camera’s CCD-shift image stabilisation system to vibrate the sensor each time the camera is shut down. The low band pass filter covering the sensor has also been given an anti-static coating of Indium Tin Oxide to prevent dust from collecting on it in the first place.

Both systems work well in the main but, although Sony’s system is less likely to allow dust to become welded onto the sensor, neither system can remove it once it’s stuck.