Exposing a sensor to a light – projects points that project to all the sensor – and so all locations on the sensor see light from all the points.
Aperture(small hole) – seeing light from only one point(inverted and reduced).
How big should the aperture be ? until diffraction happens and lights collide.
Lenses – focus so we don’t need an aperture.
Optical axis – light comes straight through and doesn’t break.
All parallel lights are bent to the focal point.
The points that do not have lights centered in a specific points are far or closes than the focus of the length are spread and known as the circle of confusion because they are not centered together but have a circle center.
There is still an aperture but only to reduce the spread and cut off some of the rays.
Thin Lense – Similar Triangles
Relation of Distance in the world z to distance z’ where the image is focused on the film/cam and f the focal length.
We can take point P with height y and project it to p’ with y’.
Using similar Triangles we can see the -y’/y = ||z||/||z’|| and -y’/y is also similar to ||z’||-f/f which is so 1/||z’|| +1/||z|| = 1/f
and any points that satisfy this equation (meaning they are z away) are in focus.