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How does a camera lens focus light onto film or an image sensor?

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Diagram showing the negative-positive photographic process: a scene is captured on film in a camera, developed into a negative, projected onto paper with an enlarger, and developed again to produce a positive print. — technology of photography Figure 1: Sequence of negative–positive process, from the photographing of the original scene to enlarged print (see text).

A camera lens focuses light onto film or an image sensor primarily through the process of refraction. Because light travels more slowly through glass than through air, it bends when entering and exiting the lens elements.12

By utilizing precisely curved surfaces—either convex or concave—a lens system redirects light rays that are diverging from an object so that they converge at a specific point, known as the focal point. When these rays are united at the focal plane, they form a sharp, real image on the film or the semiconductor device (such as a CCD or CMOS sensor) located at the back of the camera.123

Because a single lens often produces imperfect images due to aberrations, modern cameras use compound lenses consisting of multiple elements. These elements are carefully shaped and positioned to counteract each other's distortions, ensuring that light is brought to a common focus for maximum sharpness. Additionally, the aperture acts as a diaphragm to control the amount of light entering the system, which influences the final image illumination.1425