(Literally, Pre Focus.) A type of socket and lamp base designed so that the lamp can only be seated in one orientation. This was more important with the planar filaments of pre-TH lamps (as T/H lamps tend to have symmetrical filaments), but the base type remains and is primarily used today with the BTL family of lamps in most 6" Fresnels. A medium prefocus (p28s) socket. Notice the slots for two different sized fins. Medium Prefocus Lamp Socket was also the standard socket in most radial ERS s, such as Altman 360. See this thread: http://www.controlbooth.com/forums/...el-lamp-base-problems.html?highlight=prefocus. A mogul prefocus (p40s) socket used for 1000-2000W lamps. The 1000W BVT of the Altman 75Q 8" Fresnel, the 1500W DTA/DTJ of older incandescent followspots.