focus

In optics, the focus is the point where parallel rays of light, after passing through a lens or reflecting from a curved mirror, converge to a single spot. This convergence creates a sharp image. The distance from the lens or mirror surface to the focus point is called the focal length, measured in millimeters or centimeters depending on the optical system. For a simple convex lens, focal length is determined by the radius of curvature of its surfaces and the refractive index of the glass or plastic used.

Real optical systems have two principal foci, one on each side, though a lens projects an image at only one focus when light travels in a single direction. Moving an object closer to or farther from a lens shifts where its image forms, which is why camera lenses have focusing mechanisms: turning the focus ring physically moves the lens element to adjust where incoming light converges onto the sensor or film. In microscopy, the fine focus knob allows vertical adjustment of mere micrometers to place the specimen plane exactly at the image plane.

Aberrations and practical limits

Imperfect lenses do not focus all rays to a true point. Spherical aberration occurs when rays passing through the edge of a lens converge at a different distance than those passing near the center. Chromatic aberration happens because different wavelengths refract at slightly different angles, so red and blue light come to focus at different distances. These aberrations limit the sharpness and contrast of the final image and are corrected through careful lens design, using multiple lens elements and specialized glass types.

In photography and microscopy, depth of field refers to the range of object distances that appear acceptably sharp when the camera or microscope is focused at one particular distance. Stopping down the aperture (reducing its diameter) increases depth of field, even though less light reaches the sensor. This tradeoff between light gathering and sharpness is central to optical design and daily use in imaging equipment.

The term focus comes from the Latin word for hearth or fireplace, referring to the point where concentrated light or heat gathers. Early experimenters with curved mirrors noticed that sunlight reflected from a parabolic mirror would ignite kindling placed at the focus, which is why the word persists in modern optics despite its ancient origin.