Split-image Screen
A focusing screen with a central circle divided in two, whose halves align only when the subject is in focus.
In depth
The split prism turns a judgement about sharpness — which the eye does poorly — into a judgement about alignment, which it does very well. It needs a vertical or diagonal edge in the subject to work, and it blacks out on one side with lenses slower than about f/5.6, which is why it was paired with a microprism collar.
Examples in practice
A vertical doorframe appears cut and offset until focus is correct, at which point the two halves snap into one line.