Sensor cleaning

Learning goal

You will be able to recognize that dark spots that always sit in the same place in your photos come from dust on the sensor, and remove them safely. You will explain why dust becomes visible at a narrow aperture and disappears at a wide aperture; you will check the sensor with a simple dust test; you will know where dust comes from and how to prevent it when changing lenses; you will rank the camera's own cleaning vibration, the air blower and wet cleaning by their risks; you will remove the remaining spots in editing; and in a portrait, you will tell a dust spot apart from a real mark on the face, on every skin, and apply retouching only to the dust.

The question

While the roof-and-stars frame from the highland night is open on your screen, a few gray specks in the top right corner that never move, unlike the stars, catch your attention. Then you look at last week's photos: in the sky over the seaside rocks, in a portrait in front of a white wall, in a landscape you shot with a narrow aperture; the same spots, always in the same corner of the frame. In the portraits you shot with a wide aperture, though, there are none.

What are the black dots in my photos? These spots that always sit in the same place and don't move even when the subject changes are, most of the time, the shadow of bits of dust that have settled in front of the sensor. In this lesson we will talk about why dust shows in some frames and disappears in others, how to notice it, how to prevent it, and how to clean it carefully.

Context

In Lesson 4.2 we saw how grease and smudges on a phone's camera glass cast a haze over the photo. Dust and smudges on a lens's front glass usually show in the image not as a spot but as a haze or a loss of contrast, because they are far outside the zone of sharpness. Dust on the sensor is something else: it sits right in front of the sensor, a few millimeters away, and casts a sharp or soft shadow over the image.

In every camera with interchangeable lenses, the inside of the body is open to the air the moment the lens is removed, and dust can get in. That is why sensor dust is a problem almost every photographer using these cameras will face one day. Recognizing it and removing it without panic is an ordinary part of looking after the camera.

Simple explanation

Why does dust show at a narrow aperture?

In front of the sensor there is a thin layer of glass that protects it and filters some kinds of light; the dust actually settles on top of this glass and casts a shadow on the sensor. How sharp this shadow will be is decided by the aperture. At a narrow aperture, for example at f/16, the light reaching the sensor arrives like a narrow beam, and the dust leaves a sharp-edged, dark shadow. At a wide aperture, for example at f/2, the light arrives from very different angles; the dust's shadow spreads and softens, and most of the time becomes invisible.

Dust is noticed most on light, flat surfaces: a blue sky, a white wall, a studio's plain backdrop. In a busy, textured scene or in a dark area, the same dust can go unnoticed. That is why the dust problem usually shows up in landscape, product and studio photography, that is, in work where a narrow aperture and flat surfaces come together.

How is dust noticed and how is it prevented?

There is a simple way to check the sensor. Set the camera to f/16 or f/22, bring the focus by hand to a clearly blurred point, and photograph a flat, light surface, for example a cloudless sky or a blank sheet of paper. When you enlarge the frame on the computer and raise its contrast, the bits of dust show up as small, round or irregular dark spots. Doing this dust test now and then prevents surprises before an important shoot.

Most dust gets in when changing lenses. Holding the body facing down while changing the lens, making the change quickly, making the change inside a bag or a car in a windy, sandy or dusty place, and never leaving the body without a lens or a cap greatly reduce the dust that gets in. Some cameras protect the sensor by closing the shutter when switched off; you can find out from the manual whether yours does this.

How is the sensor cleaned?

The safest step is the camera's own cleaning function. Many cameras shake off loose dust by vibrating the glass in front of the sensor very fast when switching on or off. This function can also be run by hand from the menu. The second step for loose dust is an air blower: with a rubber bulb, air is blown in front of the sensor while the body faces down, and the dust falls out by gravity. Blowing with your mouth leaves droplets of saliva; compressed air sprays, on the other hand, can spray a propellant liquid onto the glass; neither is recommended.

For stuck or greasy spots that don't go with an air blower, wet cleaning done with special cleaning swabs and fluids is needed. This job needs care: the glass in front of the sensor can be scratched, a wrong move can spread the spot, and some manufacturers' warranty terms can limit cleaning by the user. If you aren't sure, leaving this job to a service center or to someone experienced is a safe choice. After cleaning, you check the result by repeating the dust test.

How are remaining spots removed in editing, and what do you watch for on a face?

Spots you notice after shooting can easily be removed with the spot removal tool of editing programs. Because dust sits in the same place in every frame, it is often possible to apply a fix you made in one frame to the other frames of the same shoot all at once. This saves time, especially in a series shot under the same sky.

In a portrait, a dust spot can fall on a face. On light skin or in front of a light backdrop, a dust spot shows more easily; on dark skin it mixes with shadow and is noticed less, but there too it can mark part of the face. When retouching, you need to tell a dust spot apart from a real mark on the face: a dust spot sits in the same place in the frame in all the frames of the same shoot, as a soft-edged gray spot; a mole, a freckle or a scar, on the other hand, moves from frame to frame together with the person. Removing only the dust leaves the face as it is, with its own marks, on every skin.

Visual evidence

On the evening you get back from the highland house, you do the camera's dust test. At f/22, blurring the focus by hand, you photograph a white sheet of paper, and you enlarge the frame on the computer and raise its contrast. In the top right corner, three round dark spots and one thin, long one show up; in the same place as the spots in the star frame.

You run the camera's cleaning function and repeat the test; two spots have disappeared, two remain. You turn the body down and blow air a few times with the air blower; the third spot goes too. The remaining thin, long spot doesn't move from its place; you decide to leave it to a service center. In the double portrait you shot last week at f/11 in front of a white wall, the spot has fallen on the forehead of your light-skinned friend; on the cheek of the dark-skinned friend beside them, there is a mole. In editing you remove only the gray spot on the forehead; the mole stays in its place, because in every frame it moves together with its owner. The same frame; the only thing removed was the dust.

Common mistakes

Summary

Dark spots that always sit in the same place in photos are, most of the time, the shadow of dust settled on the glass in front of the sensor; they sharpen at a narrow aperture, spread and disappear at a wide aperture, and show most on light, flat surfaces. A simple dust test that photographs a blurred flat surface at a narrow aperture checks the sensor; turning the body down and changing the lens quickly prevents dust. Cleaning starts from the safest: the camera's vibration, then the air blower, and only if needed and with care, wet cleaning or a service center; while the remaining spots are removed in editing, the real marks of the face are kept, on every skin.

Mini assignment

In this assignment you will check your camera's sensor and clean it if needed.

1. Set the camera to f/16 or f/22, blur the focus by hand, and do a dust test by photographing a flat, light surface and enlarging the frame.
2. If you found spots, run the camera's cleaning function and repeat the test.
3. If there are still spots, try with an air blower, turning the body down, and do the test again.
4. Remove the dust spots in a sky frame shot with a narrow aperture with the editing program's spot removal tool, and apply the fix to another frame from the same series.

Check yourself

• Were you able to find the spots in the dust test, and did you note which corner they gathered in?
• How many of the spots did the cleaning function or the air blower remove?
• When retouching a portrait, how did you confirm that what you removed was really dust, and not a mole or a freckle?

Glossary

Mini quiz

1. Why does sensor dust show more clearly at a narrow aperture?

• A) At a narrow aperture the light arrives like a narrow beam and the dust leaves a sharp shadow
• B) A narrow aperture enlarges the dust
• C) At a narrow aperture the ISO rises
• D) At a narrow aperture the dust gets inside the sensor

2. What reduces dust getting onto the sensor the most?

• A) Changing the lens in the wind while the body faces up
• B) Changing the lens while the body faces down, quickly and in a sheltered place
• C) Leaving the body without a lens
• D) Never switching the camera off

3. Which is not recommended for loose dust on the sensor?

• A) The camera's cleaning function
• B) An air blower while the body faces down
• C) Repeating the dust test
• D) Blowing with your mouth

4. When retouching a portrait, how do you tell a dust spot apart from a mole on the face?

• A) By its color
• B) A dust spot sits in the same place in the frame in all frames, while a mole moves together with the person
• C) By its size
• D) It isn't possible to tell them apart

Answers

1. A — At a narrow aperture the light arrives on the dust from a narrow angle; the shadow sharpens. At a wide aperture the shadow spreads and disappears.
2. B — Dust usually gets in during a lens change; turning the body down and keeping the change short reduces this.
3. D — Blowing with your mouth can leave droplets of saliva on the glass in front of the sensor.
4. B — Dust is fixed on the sensor and stays in the same place in the frame; the marks of a face move together with the person.

Summary card

• Dark spots that sit in the same place: the shadow of dust on the sensor.
• Sharp at a narrow aperture, gone at a wide aperture; visible on light, flat surfaces.
• Dust test: f/16–f/22, blurred focus, a flat light surface, enlarge.
• Cleaning order: vibration, air blower, and if needed, careful wet cleaning or a service center.
• In retouching, remove only the dust; keep the real marks of the face on every skin.

Bridge to the next lesson

In this module we talked one by one about the tripod, polarizing and ND filters, the remote shutter release and cleaning tools. Each of them solves a particular job; but a photographer's bag can get heavier as a new accessory is bought for every problem, and most of the things bought can end up in a drawer. What do you really need? The next lesson talks about an approach that, when buying equipment, puts knowledge first, then need, and shopping last.