You come home from a day at the coast with a card full of landscapes. Clear skies, lots of f/16. Only at home, in Lightroom, do you see them: a handful of grey blobs, in exactly the same place in every photo. That means a hundred rounds of spot removal. Sensor dust rarely shows while you are shooting, but always on the evening you want to finish a series. The good news: you can check the state of your sensor in one minute, with nothing more than a test shot and your browser. Below: what those spots are, how to take the test shot and when cleaning is really needed.
How sensor dust gets there
Sensor dust is made up of particles, think dust, pollen or a speck of grease, that get in while changing lenses and land on the filter glass in front of the sensor. You see them as blurry dark spots that sit in the same place in every photo, especially at small apertures like f/16.
Every time you change a lens, your camera mount is open and outside air gets in. That air carries dust, and some of it lands on the filter glass in front of your sensor. Zoom lenses add to it: zooming in and out moves air around, and that air has to go somewhere. On mirrorless cameras the sensor also sits fully exposed the moment the lens comes off, with no mirror in front of it.
Most cameras give their sensor a quick ultrasonic shake when you switch them on or off. That works for loose particles, but not for everything. Pollen, a grease particle or a tiny droplet sticks and stays. Those show up as blurry dark spots, always in the same position in the frame.
Why you almost never see the dust
The odd thing is: a sensor covered in dust can stay invisible for weeks. That is down to your aperture. Dust does not sit on the pixels but on the filter glass, a fraction of a millimetre above them. Shoot at f/1.8 or f/2.8 and light passes around each particle from all sides, blurring its shadow into nothing. Portrait and documentary photographers can walk around with a dirty sensor for years without noticing.
Stop down to f/11, f/16 or f/22 and the light beam narrows, so every particle casts a sharp shadow on the sensor. That is why sensor dirt always turns up in landscapes, architecture and macro, exactly the genres where you use small apertures. The depth of field calculator shows what else your aperture affects. And that is why you take a test shot at the smallest aperture your lens has: you want to see everything that is there, including what you cannot quite see yet.
How to take the test shot
You need an even surface. A white wall works, so does a clear blue sky, and a white screen at full brightness will do as well. Set your camera to aperture priority and pick f/16 or f/22, at the lowest ISO your camera has. Let the camera handle the exposure; slightly overexposing with +1 exposure compensation even helps, because dust shows up better on a bright surface than on a grey one.
Then the most important step: switch your lens to manual focus and defocus it completely. It sounds backwards, but you specifically do not want to see the wall. Any structure in the surface, from wall texture to small clouds, can be mistaken for dust. Standing close to the wall with focus at infinity works well. Feel free to move the camera a little during the exposure; whatever you photograph may blur, but the dust moves along with the sensor and stays sharp. Save as JPEG, because browsers cannot read RAW files.
In short:
- Find an even surface: white wall, blue sky or a white screen.
- Aperture priority, f/16 or f/22, lowest ISO.
- Exposure compensation +1.
- Manual focus, defocused completely.
- Save as JPEG and upload.
From test shot to verdict
You used to open a shot like this in Photoshop and drag the levels sliders together until the dust appeared. That still works, but you no longer need software. I built a free tool for it: the sensor dust check. You drag your test shot onto the page and the tool does the rest: it removes light falloff and vignetting, stretches the contrast automatically and circles every dust spot it finds.
You get a count, the size of the largest spot and a verdict: clean, slightly dirty or cleaning recommended. A slider sets how strict the measurement is, from obvious spots only to the finest dust. Click a spot and you see it magnified next to the original. Everything happens in your own browser; your photo is never uploaded. The tool also reads the EXIF data and warns you if your test shot was taken at too wide an aperture.
First aid: the blower
If the result is nothing to write home about, start with the least exciting tool there is: an inexpensive rocket blower. Select the cleaning mode in your camera menu, so the mirror or shutter moves out of the way and the sensor sits exposed. Hold the camera with the opening facing down, so loosened dust falls out instead of in, and give a few firm puffs.
Two things never to do. Do not use canned air: the propellant can condense on your sensor and leave you worse off. And do not touch the sensor with anything, including that brush that happens to live in your bag. Take a new test shot after blowing and compare. In many cases most of the dust is gone, and you simply repeat this ritual now and then.
Stuck dirt: wet cleaning or having it done
If spots stay in the same place after blowing, the dirt is stuck and blowing will no longer help. That leaves a wet clean: a swab with cleaning fluid drawn across the sensor in one movement. You can do it yourself with a kit in the right size for your sensor, but it remains precision work on the most expensive and delicate part of your camera. Scratch the filter glass and you are looking at a serious repair.
That is why I take my own cameras to Chipclean in Overasselt, here in the Netherlands: by appointment, and your camera is ready while you wait, checked afterwards as well. A professional clean costs a fraction of what a damaged sensor costs, and you know it is done right. If you live elsewhere, look for a camera service that treats sensor cleaning as a speciality rather than a sideline.
How to prevent sensor dust
You cannot prevent dust entirely, every camera collects it. But you can slow it down a lot. Change lenses with the mount facing down and out of the wind; a doorway or your coat works fine as a windbreak. Switch the camera off before changing, because a powered sensor attracts dust like a television screen. Never leave a body lying open: next lens on straight away, or a body cap. And make a habit of shaking out your bag once in a while, because more dust comes from there than you would think.
The most important habit may be the simplest one: check your sensor before it matters instead of after. A test shot every few months, and always before a big job or a trip. Then a dirty sensor is never a nasty surprise afterwards, just a bit of maintenance you saw coming.
Check your sensor now
Grab your camera, find a white wall and take that one test shot at f/16. Drop it into the sensor dust check and within a minute you know whether you can keep shooting, or whether the blower comes out of the bag. Free, no account, and your photo stays on your own device.