Extension Rings
From Nikonians Wiki - FAQs, Photo Glossary, Good Photo Locations, Help
Rings used to extend the distance between lens and film/sensor for macro or close-up work (One or several for various magnification ratios). Current Nikon extension rings are the PK11A (8mm), PK12A (14mm), PK13A (27.5mm). Sometimes also called Extension Tubes.
