Light field cameras measure not only the ray position on the sensor, but also the ray angle. I think of it as an alternative to taking a series of images of a static scene over a range of camera focal lengths or axial detector positions (time sequential imaging). All these approaches enable the photographer to get in-focus images over a range of object distances, thereby extending the depth of field.
This single-shot multiplexing is achieved with a microlens array in front of the sensor. An example I designed to illustrate the “Lytro” approach is shown below. The primary optic focuses the rays at the microlens array, which is located one microlens focal length from the sensor.
