Do you wonder why sensor dust is not visible at very large apertures, but starts to show up at small apertures like f/8? Below you will find an excellent illustration that demonstrates how aperture size affects the shape and the size of dust particles.
In short, 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 many photographers refer to this as “sensor dust”, dust actually never touches the sensor, because there is a pretty 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/8, 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 Di Luzio for sending the illustration!