I think I am quite good at math and understand basic geometry, but I have problems understanding the function of a diaphragm in cameras.
Let's say a camera is in a specific state. It captures the light from a specific "cone", going outside of the camera lens. Now, if the diaphragm moves (decreases the diameter of the hole), and nothing else moves in the camera, it can only result in a "black material" appearing around the image, right?
It will not change the size of objects in the final image, or sharpness of objects, or the "rate" (amount/time) of light, that is getting to the center of the image (which is not covered by a diaphragm). Am I right?
But if the diaphragm is never shown in the final image, and only covers rays around it, what is the purpose of it, as those rays would never make it to the image sensor anyway, right?
Edit: I think I am starting to get a clue.
My intuition was, that if there is an obstacle between two points A and B, A can not see B. The point A is on the object, and B is on the sensor, and the fact that the line between them is not straight (refracted by a lens) does not change anything.
I did not realise, that there are lots of rays (in with different directions) going from A, through the lens, and reaching B. The obstacle near the lens behaves very differently. By covering half of the lens, 2x less rays would get from A to B, but the sensor B would still receive light from A (B will still "see" A).