The first picture shows the view of an observer looking at the horizon. He sees two trees, at a distance of a few kilometers.
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What would happen in his camera, if he takes an image of the horizon? Picture 2 shows a view, seen from above, of the area he is looking at. You can see the two trees (called 1 and 2), and the camera. The light rays, coming from the trees, are shown with different colors. The camera sees the trees at different angles. So their light will be projected at different positions on the focal plane, as you can see in the enlargement of the camera. In fact, all objects on the horizon will be projected at different positions, because they all have different angles. You can also see that the trees will be projected upside down. The next picture shows how the image will be projected on the focal plane of the camera. |
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The picture only shows light falling on the center of the objective, to make it less complex. Of course light will also fall on the rest of the objective, but that light will fall on the same place on the focal plane as the light from the center.
In this example, I only used objects on the horizon. Objects in front of the horizon will also be projected, but behind the focal plane. On the focal plane, the object will be seen out of focus. This is because light rays from a non-distant object will not fall parallel to each other on the objective.