Why sensors attract dust
Every time you change a lens, the cavity behind the shutter draws air across the sensor. Even a brief exposure is enough for dust and fine fibres to settle on the low-pass filter or the sensor glass itself. Full-frame sensors have a larger surface area and collect contamination faster than cropped sensors. Mirrorless bodies, which lack a mirror box to slow air movement, can be particularly prone to dust ingestion.
What Tim checks and does
Before cleaning, Tim photographs a plain grey surface at f/16 to map the contamination precisely. Dry blower and soft brush techniques handle loose particles. Stubborn specks, including the oily deposits that sometimes migrate from shutter blades or mirror buffer foam on older DSLRs, require a wet-clean using lint-free swabs and a specialist sensor cleaning fluid appropriate to your sensor size. After cleaning a second test shot confirms the sensor is clear before the camera is returned.
When to get it done
A single prominent spot in a clear sky is enough to justify a clean, because spot-healing every image in post adds up fast. If you shoot landscapes, architecture, or any subject against a plain background, keeping the sensor clean makes a real difference to your workflow. Bring the camera body only, or with a body cap in place; there is no need to bring lenses unless you suspect a separate optical fault.