We all know that dust on a DSLR or mirrorless camera sensor can range from mildly annoying to downright problematic. You can try avoiding it, but the unfortunate truth about digital photography is that, regardless of how careful you are, dust will eventually find its way onto your sensor.
I suppose you could prevent any sensor dust by putting a prime lens on your camera when it’s brand new and never, ever taking it off. But since that’s almost certainly not going to happen, dealing with dust is a critical part of regular camera maintenance.
Which leads to a question: How do you know if your camera sensor does indeed have dust? How can you tell the difference between dust on the mirror, dust on the viewfinder, or dust on the sensor? In this article, I share several strategies to help you check for sensor dust – so that you can keep your camera sensor (and your photos!) dust-free!
Where does sensor dust come from?
Eradicating any pest requires knowledge of where it’s hiding and how it got there, so I want to start by explaining why sensor dust is such a common problem.
While it might not seem like it, the world in which we live is full of dust. Regardless of how clean your studio is or how lint-free your camera bags are, microscopic dust particles will settle on your camera at some point.
And that’s before considering outdoor use or changing lenses. When using budget zoom lenses, the simple act of repeatedly zooming in and out can “inhale” dust particles into the lens, which can then work their way into your camera over time. Once inside, they can settle on the mirror or sensor.
Additionally, changing lenses significantly increases the likelihood of dust finding its way to your sensor. (And if you change lenses in a dirty environment, the problem is compounded!)
So while there are steps you can take to minimize dust (holding the camera with the lens mount facing down while changing lenses, not changing lenses outside, etc), it’s inevitable that dust will find your camera sensor eventually. That’s why, in addition to minimizing dust exposure, it’s important that you regularly check for dust!
Identifying dust: Is it on the mirror, the viewfinder, or the sensor?
When dust gets into your camera system, it can settle in three distinct places:
- The mirror. Mirrors are only present in DSLR cameras; they sit in front of the sensor and reflect an image preview to the optical viewfinder.
- The viewfinder. Many cameras, especially high-end cameras, have viewfinders. This is the part of the camera that you put against your eye, and dust can land on the outer viewfinder window or get trapped between the inner viewfinder screen and the outer viewfinder window.
- The sensor. This is where the magic happens; it’s the light-sensing chip that converts photons into electrical signals.
It’s important to recognize that while dust can be visible if it lands on the mirror or the viewfinder, neither of these areas has any effect on your photos. While the viewfinder and the mirror are important for viewing the scene and taking the photo, mirror and viewfinder dust will have absolutely no impact on your images. You can often see these types of dust with the naked eye when you look through the viewfinder. They can be annoying, but they’re often fixed with an air blower.
On DSLRs, sensor dust won’t show itself until it leaves dark spots of varying sizes on your photos. If you can see dust when looking through your DSLR viewfinder, it’s not on the sensor.
On mirrorless cameras, sensor dust can show itself in the electronic viewfinder. However, it’s generally easier to see when reviewing your images on a computer monitor, as I explain in the next section:
Identify sensor dust by reviewing your photos
The first effective way to identify sensor dust spots is when reviewing your images. You can do this on your camera LCD, but it’s generally better to review for dust on a large monitor.
You can recognize a dust spot if it appears in the same place in multiple images, particularly in those taken at apertures such as f/8 or smaller. Dust spots are especially visible when positioned against the sky.
However, if you don’t see any dust spots when reviewing your images, they may still be present – just hidden. If you typically shoot at wide apertures, most dust particles will not show up, even against bright backgrounds. Dust may also be less noticeable in images with a lot of detail, but that doesn’t mean it’s gone.
If a dust spot is visible upon a quick glance at the image, you’re likely looking at a pretty bad spot on your sensor – one that won’t leave just because you squeeze a little air at it. You may need a thorough sensor cleaning to get rid of your worst offenders.
Whether you pay to have your sensor professionally cleaned or you are comfortable enough doing it yourself, it’s possible to overdo it. Sensors have a delicate coating that can be damaged by over-cleaning, and frequent cleaning increases the odds of damaging your sensor. So how do you do a quick and easy evaluation to see if your sensor really needs cleaning?
Identify dust spots with a test shot
Say that you’re worried about dust, but you’re not sure whether your sensor has a significant dust problem. That’s when I recommend doing a simple test.
You can run this against a clear sky, a white sheet of paper, or even your computer screen. Start by switching your camera into Aperture Priority mode. Make sure you use Matrix or Evaluative metering and the lowest possible ISO.
Turn off autofocus and dial in the smallest aperture possible (remember, a higher number equals a smaller aperture).
Fill the frame with your blank target area, manually dial the lens completely out of focus, and snap off a frame. When you open the image on your computer, look for dark spots. See any? Those are your culprits.
Confirm in post-processing
No test is perfect, and this one is no different. However, there is a neat trick you can run in Photoshop as an added layer of detection. By holding down Ctrl/Cmd + I, you will invert the image, creating a negative. The dark spots (if any) will now appear white against a dark background, making them easier to see.
In the example below, certain spots were visible in the original photo. By inverting the image, several additional trouble spots on my sensor became apparent:
Lightroom also offers a handy way to look for dust spots. Simply open your test image in the Develop module, then select the Remove tool in the upper right-hand corner (under the histogram). Check the Visualize Spots box, and Lightroom will create a high-contrast monochrome mask, which you can use to quickly identify any problematic areas!
Keep your photos pristine!
Sensor dust can generally be fixed in Lightroom or Photoshop without too much hassle, but why spend extra time in front of the computer when you don’t have to? If you strive to get things right in the camera (and you should), this is absolutely something to keep on a semi-regular checklist.
If you do find dust spots that you want to remove, there are many do-it-yourself sensor-cleaning options available. If you are comfortable doing this task on your own, great. If not – and if you aren’t, I don’t blame you – local camera shops provide sensor-cleaning services for a nominal fee.
Either way, by knowing how to quickly identify the problem, you’re in a much better position to address it and get back to taking clean, crisp, dust-free photos.
Now over to you:
Have you experienced problems with sensor dust? Do you have a good way of dealing with it? Share your thoughts in the comments below!
