|Split image spot (and microprism collar) when defocused|
image by Voxphoto (Image rights)
A split-image spot, sometimes called a split image rangefinder or split-prism, is an optical focusing aid sometimes incorporated into a reflex camera's ground glass screen. When the image is out of focus, the two halves of the image appear "split" with an offset between them. When in focus, the halves line up to form a single image. The split is typically horizontal, but sometimes diagonal.
Split-image devices are often combined with microprisms.
Some digital cameras (e.g. Fujifilm X family) offer split prism viewing using OSPDAF pixels. However, many other cameras do not have this option even is hardware supports this.