...brain cells. :-)
A friend asked me the other day: "Why is it, that dust specs on the sensor become more visible when you stop down?"
Below is my explanation, but I'm not entirely sure about it. Please leave a comment to correct me if I'm wrong.
- The dust is not really on the sensor, but on a glass filter just in front of the sensor. What you see in the photo is a shadow cast by the dust spec onto the sensor.
The shape of a shadow depend on many things, but in this case there's only one thing that change; the diametre of the light source, which is the opening of the aperture in the lens.
When the light source is very large compared to the object, the object's shadow will have very fuzzy edges, or may not appear at all. As the light source shrinks, the shadow becomes less fuzzy, and darker in the middle. I've personally tried to draw this in photoshop, so please forgive the naivistic style... :-)