Even though light is traveling in 3D space, a sensor represents it on a 2D surface. Effectively what it captures is the state of light at a particular 2D plane, which is dependent on where the lens is focused. This is something that is unique; if you change the focus of your lens and the plane that you will be selecting to capture on your sensor will change automatically. Changing the plane means, selecting a plane at a different distance from the lens. This is why focus or accommodation is said to give the depth of the object when it is focused on to it.
If you closely observe the three sequence of pictures I had in my earlier post you will understand it easily. In the first image the focus point was at the match stick, and the LED was at a distance behind it. The light rays diverging from this source from the perspective of the aperture of the lens would be a 3D cone which will be truncated at the matchstick. This is what is giving you that circular patch. As I move the focus back, this circle gets smaller and the intensity increases. The light that is reflected and diverging from the matchstick is now captured at a different plane, which makes it blur. Finally, when the focus point is moved to the plane of the led, it is recovered completely, even though it was masked by the matchstick completely from the projection perspective of the camera. Due to further increase in the distance of the focus point, the matchstick becomes even more blur.
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