Camera Lens

A camera lens, also known as a photographic lens or photographic objective, is an optical system designed to form images on a photosensitive medium such as photographic film or an electronic image sensor. Although lenses used in still cameras, video cameras, microscopes, telescopes or other imaging devices operate on similar optical principles, their construction and functional requirements vary considerably. Modern photographic lenses are often interchangeable, allowing photographers to select focal lengths, apertures and optical characteristics suited to specific applications. To achieve high-quality imaging, most lenses incorporate multiple optical elements arranged in complex groups to minimise optical aberrations that cannot be fully corrected by a single element.

Principles of Operation

Camera lenses function by collecting and focusing light from a scene onto the image plane. The simplest imaging device, the pinhole camera, uses a tiny aperture to admit a single ray of light from each point of the object. While this arrangement can form an image, it suffers from significant limitations. A pinhole that is too large produces a blurred image because each point on the sensor receives light from an extended region of the object. Conversely, reducing the pinhole size increases image sharpness only up to the diffraction limit; beyond this, diffraction further degrades resolution and severely restricts light intake.
Practical lenses address these limitations by replacing the pinhole with a convex lens positioned at a distance approximately equal to its focal length from the image plane. Parallel rays from distant objects converge at the focal point, allowing a much wider aperture to be used without loss of sharpness. The lens produces a focused pencil of rays for each object point, greatly increasing brightness while controlling the size of the blur spot.
Three important concepts in lens operation are the entrance pupil, exit pupil and aperture stop. The entrance pupil is the virtual image of the aperture as seen from the object side and determines how much light from the scene can enter the optical system. The exit pupil is the corresponding virtual image viewed from the image side. In simple lenses these coincide, but in complex designs they differ due to multiple refracting elements. Regardless of design, the lens forms an image by directing all rays entering the entrance pupil from a single object point to a single point on the image plane, provided the point lies within the field of view and the system is focused correctly.
As lenses became more sophisticated, designers introduced additional elements to reduce aberrations such as spherical aberration, coma, astigmatism, field curvature and chromatic dispersion. Although the internal construction varies, the central operational principle — the convergence of pencils of light onto the sensor — remains unchanged.

Construction and Optical Materials

Photographic lenses may contain a single optical element, as seen in simple meniscus lenses of early consumer cameras, or may incorporate more than twenty individual elements in advanced zoom lenses. Many elements are grouped together using optical cements to form compound components. The front element is particularly important because it is the first surface to interact with incoming rays and heavily influences contrast, flare and overall performance.
Modern lenses employ coatings on glass surfaces to reduce reflections, enhance colour fidelity and minimise flare. These coatings also improve light transmission and contribute to the lens’s overall optical profile. Curvatures of lens surfaces are chosen carefully to ensure that angles of incidence and refraction complement one another, controlling aberrations effectively. Achieving this balance is relatively straightforward for fixed focal length (prime) lenses but more challenging in variable focal length (zoom) systems, where compromises must be made.
Focusing mechanisms typically adjust either the entire optical assembly or selected internal groups to change the distance between the lens and the image plane. Some designs include floating elements, which shift relative positions within the lens to maintain sharpness at close range. Manufacturers use different terms for such systems, including Close Range Correction (Nikon), Floating System (Canon) and Floating Lens Element (Hasselblad and Mamiya).
Glass remains the most common material for optical elements due to its favourable refractive properties and durability. However, alternative materials such as fluorite, quartz, acrylics and other plastics are used where specialised optical behaviour is required. Plastic elements enable economical production of strongly aspherical surfaces that improve performance, although they are rarely used for the front element in higher-quality lenses due to their susceptibility to scratching.

Optical Performance and Coatings

Lens resolution is influenced by material quality, manufacturing accuracy and coating effectiveness. The 1951 USAF resolution test chart is frequently used to assess resolving power. While theoretical resolution is limited by diffraction, only a small number of high-end lenses approach this diffraction-limited performance.
Most contemporary lenses include anti-reflective coatings to reduce flare and ghosting. Many optical cements also block ultraviolet light, which reduces the need for external UV filters but may encourage fungal growth if lenses are not properly stored. Specialist ultraviolet photography often requires lenses free of cemented elements or coatings, necessitating the sourcing of uncommon optical designs.
Aperture control is normally provided by an adjustable diaphragm, though early designs used removable plates with preset hole sizes known as Waterhouse stops. Some lenses integrate leaf shutters, especially in medium-format systems, allowing precise exposure control within the lens assembly.

Focal Length and Aperture

Two fundamental parameters of camera lenses are focal length and maximum aperture. The focal length determines image magnification and angle of view: shorter focal lengths produce wider fields of view, while longer focal lengths create narrower, more magnified perspectives. Aperture size affects exposure and depth of field. It is expressed as the f-number, defined as the ratio of focal length to the effective diameter of the entrance pupil. Lower f-numbers correspond to larger apertures, yielding more light at the sensor and a shallower depth of field.
Lens assemblies often contain additional internal apertures for metering or flare reduction, and many single-lens reflex systems hold the aperture fully open until the moment of exposure to assist with bright, easily focused viewfinder images.
Focal lengths are conventionally specified in millimetres. For a given sensor or film format, lenses can be categorised according to their field of view relative to the frame’s diagonal dimension. Normal lenses have focal lengths roughly equal to that diagonal, wide-angle lenses use shorter focal lengths and telephoto lenses use longer ones. Telephoto lenses employ specialised optical configurations that allow the physical length of the lens to be shorter than its focal length, offering greater portability and optical flexibility.