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Capturing the image: Sensor cleaning

Dust is a problem with all digital single-lens reflex cameras. It can enter the camera whenever you change the lens. This dust often finds its way to the glass filter that covers the digital sensor.

The sensor is made up of millions of light-sensitive elements, each around 6 to 8μm square. It does not need a very large piece of dust or dirt to cover one or more of these elements, or pixels. If a pixel is obscured, it does not receive much light and so that portion of the image will record as grey. These grey areas are most noticeable in light-toned areas of the photograph.

Dust on the sensor shows up as grey marks on the digital image.

This problem affects film too, but in film cameras the dust is wound on with each frame. In a digital camera, the dust remains on the glass filter and affects every subsequent exposure. It is easier to retouch digital images than negatives or prints, but it can still create a lot of work if you take a large number of exposures.

Dust is less of a problem in film cameras because it is wound on with each frame.

Reducing the problem

Surprisingly, research has shown that one of the main causes of dust is the camera itself. This explains why photographers who rarely change lenses still suffer dust problems.

There are two significant camera-related areas that generate dust. One is the shutter. Every time it fires, friction between the components can create dust. Recent shutter units for EOS cameras have been designed to generate a minimal amount of dust during their operation.

The other main problem area is the plastic body cap. Every time it is attached or removed, friction between the metal mount and the plastic cap can generate dust. Since the first half of 2005, the body cap has been made of a material that produces very little dust from friction.

A body cap made of a different plastic material helps to reduce the amount of dust inside the camera.

Is your sensor dirty?

Do you have dust on the sensor of your digital camera? Can you see small grey spots in light-toned areas, such as skies?

The marks will be grey and fuzzy because the dust is not actually in contact with the surface of the sensor. Instead, dust rests on the surface of the low-pass filter that lies on top of the sensor. The gap between the sensor and the dust is enough to throw the spots out-of-focus and allow some light to creep underneath.

You can check for dust by photographing a white wall or a sheet of white paper. Use a telephoto lens, or a zoom set to its longest focal length. Do not focus on the subject - set the minimum focusing distance. Select aperture-priority (Av) shooting mode on the camera’s command dial and set the smallest aperture (f/22, for example). Now make an exposure. The shutter speed will probably be quite slow, but there is no need for a tripod, as camera movement will not interfere with the test.

Transfer the image file to a computer and view the result with any good imaging software at a magnification of 100%. You will almost certainly see some grey marks - you have to decide if these are significant. Do you regularly have large areas of light tones in your images? If so, cleaning the sensor will be worthwhile.

Prevention is better than cure

You cannot stop dust getting into your digital camera, but you can reduce the risk a little using one or more of these simple procedures.

Switch the camera off before changing the lens. This reduces the static charge on the sensor and stops it attracting dust.

Turn the camera off before removing the body cap or changing a lens.

Never leave the camera with the lens mount open. When you remove a lens, replace it immediately with another or attach the camera body cap.

Avoid changing lenses in dusty situations. If you have to change a lens in these conditions, hold the camera with the lens mount facing down to reduce the risk of dust falling into the opening.

Keep the body cap free from dust. Did you know that the body cap and lens cap attach to each other? This will keep the inside of both caps clean when a lens is on the camera.

The body cap attached to the rear lens cap to keep both clean.

Cleaning the sensor

Whatever precautions you take, one day the sensor will need cleaning. Canon approves only two methods. The first method you can try yourself using a rubber air blower. The second involves sending the camera to a Canon Service Centre.

To use the air blower technique, switch the camera to its sensor-cleaning mode. This does not actually clean the sensor - it just gives access to the sensor by raising the reflex mirror and opening the shutter blades. It is recommended that you run the camera from a mains power supply, via a DC coupler, when using this mode, so that you are not relying on battery power to keep the shutter open.

Sensor cleaning mode is accessed in the same way on all the EOS professional digital cameras. While pressing the ‘Menu’ button, select the set-up menu screen (set-up 2 menu on the EOS-1D Mark II, EOS-1D Mark II N and EOS-1Ds Mark II). While pressing the ‘Select’ button, select ‘Sensor cleaning’. Release and press the ‘Select’ button and select ‘OK’. Release the ‘Select’ button and the LCD screen will show ‘Sensor cleaning’. Press the shutter button and the mirror will lock up and the shutter will open. Check the camera instruction manual if you need further details.

The sensor-cleaning mode is accessed from the menu on the back of the camera.

The sensor-cleaning mode raises the reflex mirror and opens the shutter curtain - it does not actually clean the sensor.

The rubber blower technique is non-invasive, which means that even if the shutter closes during the procedure you are unlikely to damage the camera. All you need is a rubber blower bulb. These are often sold with a brush attachment and were originally used for cleaning film negatives and slides. Remove the brush so that you have a device that gives short jets of air as you give the bulb a sharp squeeze.

With the camera in sensor cleaning mode and the lens removed, place the tip of the blower just inside the camera lens mount and give a few bursts. This will dislodge any dust specks that are resting on the sensor. With luck the dust will be blown out of the camera.

Two or three sharp blasts from a rubber air blower will dislodge dust settled on the sensor cover.

Do not use compressed air from cans - it often contains liquid propellants that will smear the sensor.

Remember to switch off the sensor clean mode after removing dust.

Other cleaning techniques

There are several other sensor cleaning techniques. None of these are approved by Canon as there is a risk of damage to the sensor. If you try any of these methods, it will be at your own risk, and could invalidate the camera warranty.

The risk to the sensor is fairly small, because it is not actually the sensor that gets dirty. The sensor is protected by a glass filter and the dust settles on its surface. So removing the dust is rather like cleaning a lens filter - there is a risk of scratching the filter, but this risk is quite small if you follow the instructions carefully.

On the other hand, if you do scratch the sensor cover, the replacement cost will be a lot more than the cost of most lens filters.

The real danger in sensor cleaning is damage to the camera shutter. Except during an exposure, the sensor is covered by the blades of the shutter. To access the sensor, you need to open the blades. If these blades accidentally close while you are touching the sensor assembly with cleaning aids, a lot of expensive damage will occur. Do not attempt this type of cleaning unless you feel confident about your ability to do it without damaging your camera.

Brush work

While a rubber air blower will move lightly settled dust, it may not have any effect on dust that is attached more firmly. Brushing the surface of the sensor can be effective, but not with just any brush. You need one that is ultra-clean and just the right size for the sensor. Visible Dust brushes, though expensive, seem to have the right specification. They work by not merely brushing the dust away, but by using static to attract the dust onto the bristles. Ideally, a single sweep of the brush across the sensor will clear the dust. Visible Dust brushes come in different sizes to suit different sensor sizes. Read all the information at www.visibledust.com before proceeding. Remember, this technique is not approved by Canon and its use may affect your camera warranty.

A Visible Dust brush is designed for cleaning digital camera sensors.

Swabbing the sensor

You can use a plastic spatula with a clean paper cover impregnated with liquid to swab the sensor. The liquid is usually methanol - an alcohol that evaporates rapidly without leaving any residue. You apply just a few drops of methanol to the end of the swab and wipe it firmly over the sensor. The end of the spatula is flexible, which means that it is difficult to apply excessive pressure. If you want to do a second swab, just reverse the motion, so that the opposite edge of the spatula is drawn back across the sensor (this avoids any dust or dirt from the first sweep being pressed back into the surface or the glass). Sensor Swabs and cleaning fluid are supplied by Photographic Solutions Inc. (www.photosol.com/swabproduct.htm) and Just Limited (www.cameraclean.co.uk/main/guarantee.php). Remember, this technique is not approved by Canon and its use may affect your camera warranty.

Wiping the sensor cover with a swab can remove stubborn particles of dust.

Grabbing the specks

There is a useful gadget for removing larger specks of dust from the sensor cover. Called SpeckGrabber, it is a tiny piece of rubber on the end of a stick. The rubber has a built-in tackiness, which means that dust will stick to it. To use, you wipe the rubber with a special cleaning tissue and then simply touch the surface of the sensor with the SpeckGrabber. The dust particle will come away. This is a very selective cleaning method, not suitable for cleaning the entire sensor area, so you may well want to use it in addition to one of the other techniques rather than on it’s own. SpeckGrabber (from Just Limited) is not approved by Canon and might affect your camera warranty.

The SpeckGrabber is useful for removing single particles of dust.

Software solutions

Despite all your attempts at preventing dust from getting to the sensor, and various methods of cleaning, you may find that some of your exposures are spoilt by grey marks. Fortunately, it is quite easy - and a lot safer - to clean up on a computer. Simply download the images files to your hard drive and open up an image in any good imaging software, such as Photoshop Elements or ArcSoft PhotoStudio (supplied free with many EOS camera models), or Photoshop. These all have a ‘clone tool’, or similar function, that allows you to select an area close to the dust speck, and copy this over the mark. With a little skill you can retouch the image so that it is impossible to see where the spots were.

Canon’s own imaging software - Digital Photo Professional - goes one step further. It not only has a standard clone function, but also offers automatic retouching. Canon has built its ‘FARE’ system into the software - the same technology used in scanners to overcome the problems of dust. If you choose ‘Repair (dark)’ from the Tools > Start Stamp Tool menu, and then select a small area that includes the dust speck, the mark will disappear like magic. Choosing ‘Repair (light)’ will work the same magic on white spots.

Digital Photo Professional is on the EOS Digital Solutions disk supplied with a many EOS digital models, but you may need to update the software to the latest version to take advantage of this auto-retouch feature. Go to www.canon-europe.com and follow the ‘Download software’ link.

The ‘Repair’ window in Canon’s Digital Photo Professional software provides an easy way to get rid of dark spots on your photographs. Simply position the green circle over the affected area and ‘click’ the computer mouse. The size of the circle can be adjusted with the ‘Radius’ control bar to suit the mark.

New techniques to combat dust

With the introduction of the EOS 400D in 2006 and the EOS-1D Mark III in 2007, Canon has revealed new technology to combat the problem of dust. It may not have been the first to tackle the problem of sensor dust in-camera, but Canon’s solution is one of the most comprehensive, and is proving to be very effective, even in extremely hostile environments. This technology is likely to appear in other EOS cameras as they are introduced.

Repelling the particles

We talk about dust settling on the digital sensor. This is not strictly true. The sensor is a sealed unit that includes a number of filters. If dust settles, it is on the front filter. This filter is treated with an anti-static charge process to repel static-charged dust.

Not only that, but this filter is further from the sensor than on earlier models. This throws the dust specks more out-of-focus, with the result that they are less likely to show on the image.

An anti-static charge repels dust particles.

Shaking the sensor

In the EOS 400D and all digital EOS models since then, the filter in front of the sensor is attached to an ultrasonic vibrating unit driven by a piezoelectric element. When this is switched on, dust is shaken off the surface and is trapped by a sticky border surrounding the filter. The sensor assembly has an internal O-ring so that none of the dust can get between the filters or on to the sensor itself.

The filter in front of the sensor is attached to an ultrasonic vibrating unit driven by a piezoelectric element.

Although Canon is not the first with this type of anti-dust system, the camera vibrates the front filter of the sensor (low pass filter 1), rather than using an extra sheet of glass that might degrade optical performance.

The default setting of the camera operates the self-cleaning sensor for one second when the camera is turned on, and again when it is turned off. The off sequence is used because dust is more difficult to dislodge after is has been on the filter for a long period. Self-cleaning can be switched off using a menu command. You can also activate the cleaning sequence while the camera is switched on using a menu command.

Sensor cleaning never gets in the way of shooting. The moment you press the shutter button (partially or all the way), the sensor cleaning stops and you can take a picture.

The self-cleaning operation uses very little power, so it does not lead to a significant reduction in the number of shots possible from a charged battery. However, the piezoelectric element heats up during use so, to prevent overheating, it will not operate again for 3 seconds after use.

Dust Delete Data

If dust does stick, and you do not want to use a brush or swabs to clean it away, the new cameras have one more option. ‘Dust Delete Data’ maps the size and position of the particles and appends the information to the image files you capture. Then, when you open the file in Digital Photo Professional version 2.2 (supplied on CD with the camera), the software automatically erases the dust spots to give a clean image.

First, you have to obtain the Dust Delete Data. You do this by choosing the appropriate menu function and shooting a sheet of white paper that is out-of-focus. The camera overrides any settings you have made and fires at f/22 with a shutter speed of 1/2 second or faster, ISO 800, flash off, and single shot drive mode. If there is not enough light, or the lighting on the paper is very uneven, or the exposure is not adequate, a message will appear on the LCD asking you to try again.

Dust Delete Data works with both JPEG and RAW images. The data uses only a few kilobytes and does not affect the continuous shooting speed or maximum burst. The information is stored in the camera, so you do not need a CF card loaded when you capture the information. Once stored, it is applied to every image you shoot.

When you record Dust Delete Data, the date and time is also recorded, and appears on the menu so that you can check the last time it was done. It is recommended that you update the data before taking important pictures, or after changing lenses in a dusty place.

It can be a good idea to switch off the auto sensor-cleaning feature when using Dust Delete Data, or you might shake off dust that has been mapped.

You can erase Dust Delete Data, and so stop it being appended to images, by using the ‘Clear all camera settings’ menu item.

 

The EICS unit in the EOS-1D X has been improved over previous units – it now includes Ultrasonic Wave Motion Cleaning to roll dust particles down the filters in front of the CMOS sensor.

EOS-ICS Ultrasonic Wave Motion Cleaning (UWMC)

The EOS-1D X takes the self-cleaning sensor found on other EOS cameras and improves on it to make it more effective in removing dust. Instead of just vibrating the filters in front of the sensor and allowing dust to fall down under gravity to the sticky border, the camera uses Ultrasonic Wave Motion Cleaning (UWMC) that generates a carrier wave from the top to the bottom of the filters so dust particles are physically transported down to the capture surface. This is more effective, not only against large dust particles but also against the smaller particles that become obvious when the aperture is stepped down. The filters in front of the CMOS sensor also feature an anti-dust Fluorine coating that improves performance of the carrier wave with stickier or damp dust particles.