Micro prism ring
The micro prism ring (also called micro prism grid ), together with the sectional image indicator, is an aid for manual focusing (focusing, distance adjustment) of cameras . As a rule, autofocus cameras no longer have these features. If the cross-sectional image indicator allows focusing on contrast transitions, the opposite is possible with the microprism ring, focusing on almost contrast-free surfaces. The prisms break up the image in such a way that even with minimal contrasts (such as when focusing on a white wall), even slight movements of the camera cause a visible "shimmer". However, the moment the optimum sharpness is reached, the picture becomes clear and calm.
The micro prism ring is in the middle of the focusing screen in the viewfinder ; until the correct distance is set, the area in the microprism ring appears clearly blurred; this mechanism only works reliably at a light intensity of the objective of at least 1: 5.6 or better.
If available, the cross-sectional image indicator is often located in its center as an additional aid, but there are also focusing screens that have a pure central microprismatic surface.