With system cameras with interchangeable lenses, it is unavoidable that the sensor will get dirty—whether that is dust from the environment, oil from moving parts (like the mirror in an SLR camera), fibers from a cleaning brush/cloth or an unintentional fingerprint. It happens to us all, although you do not see it on every picture. Even if you never change lenses, there is a chance of the sensor getting dirty. A small number of the—generally more expensive—cameras and lenses are extra-well sealed against dust. Even that is no guarantee that no dust will reach the sensor. Dust is everywhere. If you use a zoom lens that changes length when zooming in and out, then you also suck new air—and thus dust—into the camera. The question is thus not whether your sensor will get dirty, but when your sensor will get so dirty that you will start to see it.
How do you recognize dust and other dirt on a camera sensor? Someone who wants to buy a second-hand camera should certainly know the answer to this question. How do you clean a sensor, and how do you prevent it from getting dirty again?

I don’t see anything, so is nothing there?

Anyone who has ever scanned in negatives/slides or has printed a photo in a darkroom knows how difficult it is to work dust-free. No matter how often you dust and vacuum, there is always dust in the air. It might seem as though dust would be less of a problem with digital photography. In part, that’s true: your camera sensor can get pretty dirty before you realize there’s a problem. When storing digital shots, you have fewer problems with moisture, dust and molds than for storing analog shots. And a photo printer is less sensitive to dust than a darkroom. As a digital photographer, you do have fewer problems with dust.
Until you come back from holiday: The chance is good that your first encounter with a dirty sensor will be when viewing holiday photos from Ibiza, Terschelling or your winter sport pics from Les Trois Vallées. Dirty—mostly dust—is best seen on photos made of an even subject at a small aperture (f/16 or f/22)—like a photo of a blue sky on a clear summer’s day. Holiday photo books are usually chock-full of photos with blue skies, made with a small aperture. What is that dark spot doing on my picture, and how do I get rid of it?

Bokeh & dust

Spots on the lens coating and dust in a lens are clearly visible in the bokeh rings.

“Removing” dust in Lightroom or Photoshop

Every modern photo editing program currently has the ability to remove dust and small spots by “stamping” another part of the photo over that place. Should you encounter a small bit of dust on your sensor that has been there for a while, then you discover afterwards that you have taken hundreds of pictures that have a spot. Fortunately, you do not have to remove those—if you’re lucky—in each individual photo. If the dust is at a place where there is a lot of detail, or if you have taken a picture with a large aperture (f/2.8 or smaller), then you won’t see the dust spot. The only thing that dust contributes to the photo then is a lower image quality at the place where the dust is.
Fortunately, all modern image editing programs offer the option to retouch a series of photos simultaneously. You select a series of photos, do editing with the clone stamp or the retouch pencil on one photo—in this case, removing a dust spot—and then synchronize all the selected photos. In all the selected photos, the same retouch will then be done at the place of the dust spot. Sometimes, that works amazingly well, but sometimes editing with the clone stamp or retouch pencil that worked well in the first photo works less well in the other ones. Then there is nothing for it but to manually do the extra retouches. It also happens that the dust spot is not at the same place on all your photos. In that case you will also have to retouch all your shots separately. Or just accept the dust spot.

A good photographer tries to prevent this kind of situation and does a test before taking many and/or important pictures to see whether there is dust on the sensor. How do you do that, and what do you see then?

What is on my sensor?

Dust on the sensor: with a thick sensor stack (Olympus, Panasonic), the flecks are less sharp than with spots on the sensor of a Nikon/Canon SLR camera with a thin sensor stack.	Hairs and fibers on the sensor
Damaged coating in front of the sensor due to the application of the wrong cleaning fluid.	Grease or oil on the sensor smeared across the sensor after a dry cleaning
Spit on a sensor: the result of blowing away a particle of dust on the sensor, with multiple, difficult-to-remove spots as a result. For removing spit, you need a different cleaning fluid than for a regular wet sensor cleaning.	Removing dust from the camera with compressed air carries the risk that droplets of propellant can form on the sensor. These droplets cannot be removed with a bellows or a brush. If you try to remove this with a brush, then you smear the substance across the sensor. Do not use compressed air that is not intended for the removal of dust, and do not hold the compressed air can upside down. Shaking before use can also be the cause of droplets being formed on the sensor.
Thanks to ChipClean for making these photos available.

Testing: Photoshop recipe for checking for dust on the sensor:

• Dirt on the sensor will best be visible when using a lens with a short focal length (wide angle). Due to vignetting, this kind of lens is sometimes less suited for checking for dirt on the corners of the sensor. If you have trouble with vignetting, then choose a lens with a longer focal length.
• Set the camera to a pre-set aperture (A) and choose as small an aperture as possible (f/22).
• Choose RAW or jpg in the highest resolution and adjust the white balance as needed for artificial light.
• ISO value: ISO 100 or 200
• Auto focus off, lens focused to the shortest possible distance. Take a picture of a piece of light (preferably blue), smooth paper or of an even grey or blue sky.
• Make sure that you are not sharply focused on the paper by getting as close to it as possible. As a finishing touch, you can move the camera while taking the shot. If the paper is still sharply in focus, then focus to infinity.
• Open the shot in an image editing program (Photoshop, but another program will work just as well) and view the image at 100% enlargement. Increase the contrast, for example by adjusting the levels, so that dirt on the sensor is easier to spot.

Lightroom recipe for making dust on a sensor visible:

Making dirt on the sensor visible is a breeze in Lightroom:

• Take a picture in the same way as in the Photoshop recipe.
• Import the shot into Lightroom, go the Develop menu (1), choose Retouch pencil under the histogram (2) and click below the photo on the left on “Visualize spots”. With the slider next to the selection area, you can set the sensitivity. Often, you can slide that all the way to the right.

What kind of dirt is on my sensor?

Tape residue on the sensor, as a result of an attempt to clean the sensor	Residue from wet cleaning
Scratches on the sensor, for example as a result of improper cleaning.	Mold on the sensor due to long-term storage of the camera under damp conditions
No dust on the sensor, but a hot pixel. You do not resolve this by cleaning the sensor, but many camera brands offer the option of saving a reference photo, after which an automatic correction is done for a hot pixel.	Dust on the back of the lens; these spots are much larger and more vague than the spots from dust on the sensor
The chance is good that multiple dust particles and fibers will appear in frame when you do a check of your own camera sensor. If they come clearly into view, or if there are many of them, then it is time to clean the sensor. We will get into the question of how you can prevent dust on the sensor, and whether all camera brands are equally bothered by dirt on the sensor (the answer is: no) in a subsequent article: Preventing dirt on the sensor.
Thanks to ChipClean for making these photos available. In two follow-up articles, you will see how you remove dust and other dirt from the sensor and how you prevent a dirty sensor.