Grades 3–5 · Machines
How does a camera lens focus an image?
A camera lens focuses light by changing the directions of incoming light rays. Rays are lines used to show how light travels. Light from one point on an object spreads out, but a converging lens can redirect some of that light so it meets at an image point behind the lens.
In the diagramA lens bends light toward an image planeObjectLensSensorSharp planeNearer objectFarther sensorTwo rays from one object point are shown.The sensor sits where these rays meet.
A camera's sensor is the surface that records the image. To make the selected object sharp in this simple drawing, the sensor must be at the plane where its image points form. If the sensor is too near or too far from the lens, rays from a point have not met, or have already spread apart again, when they reach it.
For the objects shown, moving the object farther away brings its image plane closer to the lens. The nearer object therefore needs the farther sensor position. Real cameras often focus by moving lens parts rather than the sensor, but the aim is the same: bringing the image into focus on the recording surface.
About this explanation
Starting knowledge: Trace a light path and compare near and far objects.
Sources
- OpenStax, Rice University — Thin Lenses