I'm curious about how the aperture of a lens regulates the amount of light entering the film/sensor. When the aperture is stopped down, why does the image size still stays the same and the light entering is less? Can anyone answer this question? Thanks.
The blades have the effect of making the lens have a smaller diameter. In the very early days of photography the "stop" used to be placed in front of the front element. It was a metal plate with a set of holes cut in it and the photographer choose the hole.
Later optical engineers found a way to place the stop inside the lens but at a specific location where it would have the effect of the old exterior stop. They did it by placing the new adjustable stop in a place in the lens where there is no image plane. It is hard to describe but the are places inside a lens system where any given light ray can be associated with any part of the image
It's clear that in front of the lens this must be true because every point on the subject can "see" every point on the surface of the front lens element. So cutting off some plart of the front element does not cut off some part of the image, it only darkens the image over all. There are other places were this is true too and this is where you can put a diaphram to control the aperture. Dust acts the same way.
Dust on a lens surffce does not cast a shadow, but only blocks light. One other way to think about it is that the dust (or aperture) is so grossly out of focus that it casts it's very soft shadow over the entire image.