Conventional cameras are instruments to get images of real objects. The photography is gotten collecting on a film, through a lent, the light spread by the objects of the scene. In the digital cameras, that last is substituted by an array of sensors (CCD) which allow to measure the amount of light for each pixel. In order to understand the physics behind the plenoptic camera, we have to define: the focus and the depth of field. In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge; the depth of field (DOF) is the distance between the nearest and the farthest objects that are in acceptably sharp focus in an image.