How to tell if it's sensor dust
Take a test photo of a plain, bright, even surface — a clear sky or a white wall — at a small aperture like f/16 or f/22. Sensor dust appears as soft dark blobs in the same position in every frame, and becomes more obvious the smaller the aperture.
If the spots move when you change or clean the lens, they're on the lens or filter, not the sensor. Marks that only appear in one shot are usually dust in the air, not on the sensor.
Why professional cleaning is safer
The sensor's surface (the low-pass filter or, on mirrorless, the exposed sensor) is delicate. Wet cleaning with the wrong fluid or technique can smear or scratch it, and blowing with canned air can drive dust deeper or spray propellant onto the sensor.
Professional cleaning uses the correct tools in a dust-controlled way, with a test shot afterwards to confirm the sensor is clean. Mirrorless sensors are more exposed, so careful handling matters even more.
How often is realistic
There's no fixed schedule. Clean only when you actually see spots in your images, or before an important shoot if you change lenses often in dusty conditions. Over-cleaning is unnecessary and adds handling risk.
Confirm it's the sensor:
- Shoot a plain bright surface at f/16–f/22
- Spots appear in the same place in every frame
- Spots don't move when you change the lens
- More visible at smaller apertures
- Bring it in for a controlled clean and a confirming test shot