Microprism Collar
A ring of tiny prisms around a focusing screen's centre that shimmers when the image is out of focus and clears when it is sharp.
In depth
Unlike a split prism, a microprism collar works on any subject texture rather than requiring a straight edge, and it complements the split image at the centre of the same screen. It also darkens with slow lenses, and modern screens optimised for autofocus brightness usually omit both aids.
Examples in practice
Focusing on a face, the shimmering texture across the cheek resolves into smooth skin at the point of sharpness.