Changing the focal length of a camera by adjusting the physical zoom lens. All zoom lenses in film cameras and digital single lens reflex (DSLR) cameras are optical zoom. Digital point-and-shoot cameras as well as consumer and prosumer video camcorders have optical zoom, but they also have digital zoom.
Digital Is Software Zoom
At the telephoto side of the camera, the computer takes over and interpolates the results. For example, a still camera may have 3x optical zoom in the lens, but can process the image with its internal computer to 10x. Depending on the camera and subject matter, the software-created digital zoom can be quite acceptable or mediocre.
The Higher the Optical, the Better
The higher the optical zoom, the better the results. In a still camera, an optical zoom of 10x or more is called a "superzoom" lens. Note that if a still camera is advertised with "10x zoom," that does not necessarily mean optical zoom only. In many cases, the optical is 3x with the additional 7x being digital.
With camcorders, the optical zoom typically ranges from 10x to 64x, while the digital zoom can go from 100x to 1000x. See digital camera