This is often used in combination with a prime focus t-adapter. A focal reducer as it's name suggests reduces the focal length of the lens system in a telescope .A Focal Reducer lens is used mostly with a CCD or film camera. A focal reducer works by concentrating the light cone produced by the scope into a smaller, yet brighter circle. Because the light cone at focus is actually smaller and the detector stays the same size, the field of view also increases. A wider field of view allows relatively large objects to fit within the frame of a tiny CCD chip.
Because the focal ratio of the lens or telescope is reduced, the time required to image an object is also reduced. When imaging faint objects, the integration time is a significant factor, and a significant reduction is achieved by employing a focal reducer.