How a lens works: a 3D explanation

Read the video’s visual description

Light reaches a point on the tree’s surface and reflects in many directions. The portion reaching the lens is refracted and brought together. Light from each point forms its own small image point; together these produce an inverted tree on the sensor.

When the tree is in focus, light from one of its points comes together at the sensor. Light from the distant mountain crosses before reaching the sensor and then spreads again, blurring the mountain. The diagram distinguishes the tree’s and mountain’s image positions from the sensor plane.

The lens moves while the sensor stays fixed, bringing the mountain’s image position onto the sensor. The mountain becomes sharp. Light from the tree now reaches the sensor before converging, so one tree point makes a large circular blur. Focusing changes which subject distance appears sharp.

The animation separates the tree’s rays from the mountain’s and narrows the diaphragm. A narrower transmitted bundle makes a smaller blur circle on the sensor for an out-of-focus point. The diaphragm changes the range of admitted rays, rather than moving the ideal image position.

A schematic lens changes curvature to show the relationship between light bending, focal length, image size and field of view. Less pronounced curvature gives weaker bending and a longer focal length. This illustrates an optical relationship; it does not mean that the glass in a real zoom lens changes shape.

A moving three-dimensional view shows that only the light directed toward the opening enters the lens. The circular area over which the whole scene can be imaged is the image circle; the image does not extend equally beyond it. This overall image area is different from the circle of confusion, which describes the blur from one point.

A rectangular sensor outline is placed over the image circle, and only its interior is recorded. As the rectangle grows beyond the circle, its corners become dark. Light paths, lens travel, image positions and blur are schematic representations made easier to see, not the literal construction or dimensions of a real camera.

The final scene returns from a lens to a pinhole. Sunlight travels straight through a small opening and illuminates the inside of a box. As the sun’s direction changes, the illuminated position and surface change. Observe the projected light inside the box; never look directly at the sun or at it through the opening.