Shooting at relatively wide aperture, you may not even know your sensor has dust. Suddenly you get the opportunity to shoot some macro images. Excitedly, you open the RAW files in your processing software only to realize you have the dreaded “sensor dust”, the bane of every macro photographer. Over time, especially with heavy lens changes, or, in my case, extremely dirty fieldwork, the “sensor” of a DSLR can become dusty. Actually, “sensor dust” is a bit of a misnomer: the dust is not actually on the sensor, but on the filter that coats the sensor. If you want to check whether your (DSLR) body is infested, and how badly, stop the aperture down very tight, say f16, and take a photo of a white surface. Make sure you are sitting down when you open the resulting image in your processing software… Opinions on the best way to clean sensors range far and wide, and there are a variety of resources out on the web as to how you, the user, can clean the sensor without sending back to the manufacturer.
Acropora samoensis image with heavy sensor dust visible on the right side of the image