Another reader of ours, Frank Di Luzio, sent the below image that explains exactly why sensor dust is more visible at small apertures. While I have explained this phenomenon to some of our readers before (see the comment section), I have not had a chance to write a separate article with a proper illustration, demonstrating how aperture size affects the shape and size of dust particles. Thanks to our generous readers like Frank, I now do not have to do it, because the below illustration is perfect.
In summary, when the size of aperture is large (a small F-number like f/2.8), light rays reach dust particles that are sitting on the sensor filter from different angles. Remember, although I refer to this as “sensor dust”, dust actually never touches the sensor, because there is a thick filter (actually, more like a number of filters packed together to form a single filter) that sits in front of the camera sensor. Therefore, by the time light reaches the physical sensor, it is spread out on a very large area, making dust appear as a large blob with a soft ring. When using very large apertures like f/1.4 on fast prime lenses, these blobs might be so washed out that they might be practically invisible to your eye. That’s why portrait photographers notice dust less often than landscape photographers!
Now when the lens is stopped down and aperture is significantly smaller, say at f/16, light rays coming from the lens diaphragm are perpendicular to the sensor filter. Because the angle is more or less straight, dust specks also cast direct and defined shadows on the sensor. That’s why dust shows up in images much smaller, darker and with more defined edges at small apertures.
Big thanks to Frank for sending the illustration!