VIDEO
Pinhole camera: a 3D explanation
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A 3D view follows light reflected from points on a tree through a small hole to an inverted image. It compares how small and large openings change each point’s footprint and the clarity of the image.
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Sunlight reaches the tree and reflects from each point on its surface in many directions. Different colors distinguish light from the top, middle and bottom of the tree. The rays spread in depth as well as across the screen.
A dark box with a small opening stands between the tree and the screen. Light that strikes the wall stops there; only the portion directed toward the opening enters. Light from different points passes through at the same time. The opening neither bends nor converges the rays.
Light continues straight after passing through the opening. Light from the top of the tree reaches the bottom of the screen, light from the bottom reaches the top, and light from the middle reaches the middle. The resulting tree image is inverted vertically and horizontally.
The viewpoint travels along the light path: from the sun to the tree, from reflection at the tree toward the opening, and through the opening to the screen inside the box. Outside and inside views show that the path remains straight.
With a small opening, light arriving from one point on the tree makes a small circular footprint on the screen. More footprints, corresponding to different points, gradually appear and combine to form the fine outline of an inverted tree.
The opening grows while the tree and screen stay in place. The footprint from the same point becomes larger. Large footprints from many points overlap and blur the outline. The final side-by-side comparison connects the size of each footprint with the appearance of the whole image.
The rays and opening are enlarged schematic representations, and the circles explain how the image forms. Brightness is adjusted for the comparison. A smaller opening does not make the image indefinitely sharper: diffraction also causes blur when the opening is too small.