"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!"